Use info dialog to display per-album error log.

When an error occurs during loading of a release, it will be visible
in info dialog for this release (which was unused).

New icon was added to mark visually releases with errors, and scrollbars
will appear in info dialog if needed.
Make modified releases a bit more obvious using a specific icon.
Display cover art errors associated with album if any
Switch tab title to Errors when needed.
Merge pull request #127 from zas/album_error

Improve feedback regarding album status