Fix convert plugin attempting to process a non-media file

[pierreay]

Description

My library is managed using Beets for organization and git-annex as storage backend. Therefore when using this system, while my library files always exists on my filesystem, some files may be empty (without content). In this case, when I'm running the convert plugin, I don't wants it to process files which are empty (same apply for any Beets plugin). Hence, I added a check that the file is readable as a MediaFile before doing any process.

Before this fix, trying to encode an empty file would have lead to an error while leaving convert doing its side-effects and convert would also copy empty files to destination for files that doesn't need to be re-encoded.

In my case, this is empty files, but the problem can be anything else (depending on the storage backend) and/or corrupted files. Conclusion, I think checking that the file is readable is always recommended before proceeding to heavy operation like this.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)