Incorrect credentials cause a crash while creating `Credentials`.

**Does this issue affect the google-cloud-cpp project?**
Yes

**What component of `google-cloud-cpp` is this related to?**
Authentication
**Describe the bug**
Providing an incorrect credential file causes a crash.

**To Reproduce** Steps to reproduce the behavior:

1. Call `::google::cloud::MakeServiceAccountCredentials` with unparsable file contents.
2. It will crash.

**Expected behavior**
There should be no crash.