Python 3.7.x has changed the error for invalid base64 payloads a couple

of times. This means that a developer testing against one version of

Python 3.7.x might see tests pass while a developer testing with another

version might see them fail.
Instead of testing the full, specific string, we're now testing only the

beginning of the string, so we can ignore the specific part of the error

that keeps changing. This is done by moving to assertRaisesRegexp()

instead of assertRaisesMessage().

Unit tests pass on Python 3.7.0 and 3.7.3.