Errors from http_listener should match InfluxDB

[danielnelson]

Relevant telegraf.conf:

[[http_listener]]
  service_address = ":9999"

System info:

N/A

Steps to reproduce:

1. Send malformed line protocol to http_listener
   • cpu value=
   • cpu value=42\ncpu value=

Expected behavior:

Telegraf response should have X-Influxdb-Error, while the error strings do
not need to be identical they should contain the prefixes unable to parse
and partial write:

$ echo -e "cpu value=" | curl -v 'http://localhost:8086/write?db=telegraf' --data-binary @-
< HTTP/1.1 400 Bad Request
< Content-Type: application/json
< X-Influxdb-Error: unable to parse 'cpu value=': missing field value
<
{"error":"unable to parse 'cpu value=': missing field value"}

$ echo -e "cpu value=84\ncpu value=" | curl -v 'http://localhost:8086/write?db=telegraf' --data-binary @-
< HTTP/1.1 400 Bad Request
< Content-Type: application/json
< X-Influxdb-Error: partial write: unable to parse 'cpu value=': missing field value dropped=0
<
{"error":"partial write: unable to parse 'cpu value=': missing field value dropped=0"}

Actual behavior:

$ echo -e "cpu value=" | curl -v 'http://localhost:9999/write?db=telegraf' --data-binary @-
< HTTP/1.1 400 Bad Request
< Content-Type: application/json
< X-Influxdb-Version: 1.0
<
{"error":"http: bad request"}

$ echo -e "cpu value=84\ncpu value=" | curl -v 'http://localhost:9999/write?db=telegraf' --data-binary @-
< HTTP/1.1 400 Bad Request
< Content-Type: application/json
<
{"error":"http: bad request"}

[danielnelson]

Errors are also printed to the Telegraf log, these should be removed or logged only at debug level:

2018-09-24T23:36:27Z E! metric parse error: expected field at offset 23: "cpu value=84\ncpu value=\n"
2018-09-24T23:37:37Z E! metric parse error: expected field at offset 10: "cpu value=\n"