Improve code coverage for CurlClient. (#1744)
Improve code coverage for CurlClient. (#1744)

I thought the existing tests would cover the libcurl failure cases,
obviously I was wrong. This reuses the tests for credential failures
because otherwise we would need to rewrite basically the same tests.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp