Strip excess custom exceptions from 'gcloud.streaming'

[tseaver]

Many (all, maybe) of the custom exception types in gcloud.streaming.exceptions are providing no value, and could be replaced with either standard exceptions (ValueError, TypeError, etc.), or one of our existing types mapping particular HTTP response codes.

Strip out as many of the custom types as seems feasible, and move the remainder to gcloud.exceptions.

[dhermes]

👍