Check failures exit with 0

[tomjkidd]

Talked briefly with @danielcompton privately, describing a situation I ran into in the wild using kibit in our CI. When check failures occur, their exceptions are caught and logged to stderr, but the exit code is 0. This makes it so that our tests pass, and we were unaware that there were things we needed to correct. I created a repo to provide the simplest project that demonstrates the issue (using a simple require statement).