Removing all uses of self.fail().

[dhermes]

Replacing instead with assertRaises statements.

[dhermes]

This is towards #212. Amazingly enough there are still lines in tests that never get executed / have branch misses:

tests/contrib/test_multistore_file.py : 108->110, 113->-112, 158->-138
tests/test_client.py                  : 311->-311, 329->-329
tests/test_file.py                    : 83->-82, 180->-180, 241->-226
tests/test_service_account.py         : 102->106

Though the test_service_account.py one is a bug in coverage.

[nathanielmanistaatgoogle]

Looks great; thank you for the clean-up and merge when you please.