google-cloud-python: Add Error Reporting gRPC

This is a tracking work item to get the Error Reporting GAPIC into this library:

https://pypi.python.org/pypi/gapic-google-cloud-error-reporting-v1beta1

@steren and I agree that the existing (small) report and report_exception is what we want to keep, along with possible more framework integrations in the future. So I think what we primarily want is the same thing as with done with Logging where we add the GAPIC as a dependency, and then fallback to the existing code if gRPC is disabled.

One small question is if and to what extent we want to expose the rest of the GAPIC besides events.report via this library. There are a few API calls that will likely only be used by power users, but it might be worth having some sort of pass-through mechanism. Unless I’m mistake, it looks like so far we either wrap all the GAPIC calls or pass-through completely, but we haven’t done both, so not sure exactly how this would look.

cc @bjwatson @jgeewax @omaray in case you have comments.