Restore audience mismatch error messages

A recent change caused audience mismatch error messages to go from this:

    "audience mismatch: domain mismatch"

To this:

    "audience misatch: Error: domain mismatch"

This repo didn't have tests for the error message, but browserid-verify does and they started failing when I updated the dependency there.  This PR restores the messages to their previous value, which IMHO is nicer.  It also adds some tests and fixes a test that should have been failing, but wasn't.

@fmarier r?