Exit status should be non-zero if some file(s) couldn't be written to

[Shados]

I've come across some strangely-encoded* FLAC files in my library which loudgain gave a "Couldn't write to" error for. However, because loudgain doesn't exit with an error status in such cases, I only detected the issue well after the fact, while I was separately checking files for the presence of RG tags.

If loudgain were instead to exit with a non-zero code when encountering issues like this, wrapper scripts like rgbpm would be able to sanely detect and report issues to the user.

---

*: Somehow the original encoder managed to have a metadata block type other than STREAMINFO as block 0, which is I presume what caused loudgain to refuse to write to the file.