Errors don't crash test runner

[refactorized]

This is more of a feature than an issue - normal tape now crashes the test runner on thrown errors, but blue-tape seems to retain the old behavior, and that of tape-catch.

This should be called out in the readme, as both tape-catch and blue-tape are wrappers, and mutually exclusive. Accordingly, it would seem like a developer would need to choose between exceptions as failing tests, or promises, but they can really have both with blue-tape.