Exit code with diff and ignore

the docs state:

> If --diff is supplied, YAPF returns zero when no changes were necessary, non-zero otherwise (including program error). You can use this in a CI workflow to test that code has been YAPF-formatted.

But yapf also throws an exception and returns non-zero when it doesn't find any files. It might be nice to know if you specified a non-existent file, but yapf also returns an error code if all files are ignored due to .yapfignore or the equivalent command line argument.

Start by tracing the command-line handling for --diff, missing files, and files excluded by .yapfignore or its command-line equivalent. The issue does not name a file or test and does not settle the desired exit-code behavior, so clarify the expected distinction before implementing and add coverage for the agreed cases.

Written by the indexing model from the issue text.