Catch UnreadableFileError when reading files in a (try_)write context
Catch all errors when loading state file.

A crash during the multi-threaded import process may leave the pickled
state invalid (see #913). We recover from all these errors.
mediafile: Catch IndexError when file has empty list tag

Fixes #913
lyrics: Use multiple lyrics search strings.

In particular we use the original artist and title before stripping
*and* and *featuring* suffixes.

Fixes #914.
Debug log for unreadable state files (#913)
Merge pull request #910 from andriykohut/master

Fix problems with new discogs_client version
Changelog for #910 (fix #915)
Merge pull request #912 from dreewoo/UnreadableFileError

Catch UnreadableFileError when reading files in a (try_)write context
Changelog/thanks for #912
convert: Catch errors when writing tags

Fixes #878.
Refactor modfiy command and tests

Parsing of the modify command line is handled by `modfiy_parse_args()`.
Test use the command line.