Remapping (almost) all RPC status codes to our exceptions in datastore.

[dhermes]

Fixes #2497.

(Relevant to #2583 as well.)

FYI @jonparrott

[dhermes]

@jonparrott @daspecster PTAL

[daspecster]

I like this better IMHO.

[dhermes]

FYI I am toying with Circle CI. Looks like Travis is being helpful for the time being though

[tseaver]

Sorry to be late to the party. Wouldn't the _GRPC_ERROR_MAPPING and _grpc_catch_rendezvous bits logically live in cloud/google/cloud/exceptions.py? I presume we might want to use them in other APIs.

[dhermes]

@tseaver I intentionally didn't do that, mindful of two things:

• it may be premature to lift into core (i.e. YAGNI, but not quite YAGNI, more like YAGNI-yet)
• changes in core have different implications release-wise than in the service packages

[tseaver]

@dhermes FWIW, anywhere we are currently sniffing at StatusCode STM to be a candidate:

core/google/cloud/_testing.py
core/google/cloud/streaming/exceptions.py
core/google/cloud/streaming/http_wrapper.py
core/unit_tests/streaming/test_http_wrapper.py
datastore/google/cloud/datastore/_generated/datastore_grpc_pb2.py
datastore/google/cloud/datastore/connection.py
datastore/unit_tests/test_connection.py
docs/error-reporting-usage.rst
error_reporting/google/cloud/error_reporting/client.py
error_reporting/unit_tests/test_client.py
logging/google/cloud/logging/_gax.py
pubsub/google/cloud/pubsub/_gax.py

[dhermes]

OK.