lint: Enable errcheck, fix failures

[abhinav]

This enables linting with errcheck on the repository.
Exclusions were added for functions that are known
to never fail, e.g. all Write methods on Zap's buffer.Buffer.

In attempting to enable exclusions for these functions,
I discovered and fixed a typo in the golangci.yml.

Complete list of issues fixed

encoder_test.go:44:18: Error return value is not checked (errcheck)
encoder_test.go:55:18: Error return value is not checked (errcheck)
error.go:64:19: Error return value of `arr.AppendObject` is not checked (errcheck)
http_handler.go:83:13: Error return value of `enc.Encode` is not checked (errcheck)
http_handler.go:88:14: Error return value of `enc.Encode` is not checked (errcheck)
http_handler.go:92:13: Error return value of `enc.Encode` is not checked (errcheck)
http_handler.go:95:13: Error return value of `enc.Encode` is not checked (errcheck)
http_handler_test.go:170:24: Error return value of `res.Body.Close` is not checked (errcheck)
logger.go:377:24: Error return value of `log.errorOutput.Sync` is not checked (errcheck)
sink.go:69:17: Error return value of `sr.RegisterSink` is not checked (errcheck)
stacktrace_ext_test.go:178:13: Error return value of `os.MkdirAll` is not checked (errcheck)
writer.go:65:11: Error return value of `c.Close` is not checked (errcheck)
writer_test.go:94:11: Error return value of `os.Remove` is not checked (errcheck)
writer_test.go:258:9: Error return value of `w.Write` is not checked (errcheck)
zapcore/buffered_write_syncer_bench_test.go:43:15: Error return value of `w.Stop` is not checked (errcheck)
zapcore/buffered_write_syncer_bench_test.go:47:12: Error return value of `w.Write` is not checked (errcheck)
zapcore/buffered_write_syncer_test.go:104:11: Error return value of `ws.Write` is not checked (errcheck)
zapcore/core.go:107:9: Error return value of `c.Sync` is not checked (errcheck)
zapcore/core_test.go:151:13: Error return value of `core.Write` is not checked (errcheck)
zapcore/encoder_test.go:288:17: Error return value of `enc.AddArray` is not checked (errcheck)
zapcore/encoder_test.go:316:17: Error return value of `enc.AddArray` is not checked (errcheck)
zapcore/encoder_test.go:723:14: Error return value of `mem.AddArray` is not checked (errcheck)
zapcore/entry.go:245:23: Error return value of `ce.ErrorOutput.Sync` is not checked (errcheck)
zapcore/entry.go:257:22: Error return value of `ce.ErrorOutput.Sync` is not checked (errcheck)
zapcore/error.go:101:19: Error return value of `arr.AppendObject` is not checked (errcheck)
zapcore/json_encoder_bench_test.go:34:16: Error return value of `enc.AddObject` is not checked (errcheck)
zapcore/json_encoder_bench_test.go:98:16: Error return value of `json.Marshal` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:252:16: Error return value of `e.AddObject` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:260:16: Error return value of `e.AddObject` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:268:16: Error return value of `e.AddObject` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:292:13: Error return value of `e.AddArray` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:423:21: Error return value of `arr.AppendObject` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:431:21: Error return value of `arr.AppendObject` is not checked (errcheck)
zapcore/json_encoder_impl_test.go:533:20: Error return value of `arr.AppendObject` is not checked (errcheck)
zapcore/level_test.go:162:16: Error return value of `l.MarshalText` is not checked (errcheck)
zapcore/memory_encoder_test.go:221:16: Error return value of `e.AddObject` is not checked (errcheck)
zapcore/memory_encoder_test.go:235:16: Error return value of `e.AddObject` is not checked (errcheck)
zapcore/memory_encoder_test.go:288:54: Error return value of `e.AppendReflected` is not checked (errcheck)
zapcore/memory_encoder_test.go:294:18: Error return value of `e.AppendArray` is not checked (errcheck)
zapcore/memory_encoder_test.go:305:18: Error return value of `e.AppendArray` is not checked (errcheck)
zapcore/memory_encoder_test.go:306:24: Error return value of `inner.AppendObject` is not checked (errcheck)
zapcore/memory_encoder_test.go:321:18: Error return value of `e.AppendArray` is not checked (errcheck)
zapcore/memory_encoder_test.go:322:24: Error return value of `inner.AppendObject` is not checked (errcheck)
zapcore/tee_test.go:123:13: Error return value of `tee.Write` is not checked (errcheck)
zapcore/write_syncer_bench_test.go:40:12: Error return value of `w.Write` is not checked (errcheck)
zapcore/write_syncer_bench_test.go:54:12: Error return value of `w.Write` is not checked (errcheck)
zapcore/write_syncer_bench_test.go:67:15: Error return value of `w.Stop` is not checked (errcheck)
zapcore/write_syncer_bench_test.go:71:12: Error return value of `w.Write` is not checked (errcheck)
zapcore/write_syncer_bench_test.go:86:12: Error return value of `w.Write` is not checked (errcheck)
zapcore/write_syncer_test.go:73:9: Error return value of `ws.Sync` is not checked (errcheck)
zaptest/observer/observer_test.go:176:15: Error return value of `logger.Write` is not checked (errcheck)

[codecov]

Codecov Report

Merging #1345 (b86843b) into master (b900128) will increase coverage by 0.44%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1345      +/-   ##
==========================================
+ Coverage   97.58%   98.03%   +0.44%     
==========================================
  Files          52       52              
  Lines        3392     3403      +11     
==========================================
+ Hits         3310     3336      +26     
+ Misses         71       58      -13     
+ Partials       11        9       -2     

Files Changed	Coverage Δ
error.go	100.00% <100.00%> (ø)
http_handler.go	100.00% <100.00%> (ø)
logger.go	96.55% <100.00%> (ø)
sink.go	100.00% <100.00%> (ø)
writer.go	100.00% <100.00%> (ø)
zapcore/core.go	93.61% <100.00%> (ø)
zapcore/entry.go	98.33% <100.00%> (+8.33%)	⬆️
zapcore/error.go	86.27% <100.00%> (+0.85%)	⬆️

... and 1 file with indirect coverage changes

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