More Nuanced Return Codes

Hello,

This is a proposal to change the manner in which beets uses return codes. For automation purposes it may be useful to provide non-zero return status even when beets itself executed without error but when operation requested did not succeed. Here are the scenarios I can think of where I might want a different return status:

Beets returns success
Beets failed to execute correctly
Import/Remove/Update/Move/Write failed
List returned 0 results

There may be more and this list isn't meant to be exhaustive.

Thanks,
Mike

No files, tests, or entry points are named. Start by tracing how the CLI currently reports success and failure for import, remove, update, move, write, and list operations, then clarify the return-code policy for each outcome and identify the tests needed to define the expected statuses.

Written by the indexing model from the issue text.