fix: Improve transcoding error message (#442)

* fix: Improve transcodding error message

This fixes https://github.com/googleapis/python-api-core/issues/441 and https://github.com/googleapis/python-api-core/issues/440. Also, with this change we stop outputting the whole request message in transcodding erro message to prevent leaking any confidential information from a request message in a form of an error in a log message.

* reformat code with black
2 files changed