Fix capitalization on Backgrounds subsystem, once more.
Improve songparser logging.

- Multi-line messages for better readability.
- Silent exceptions (mostly about license.txt being not a valid UltraStar format song) are now logged (debug level, obviously).
- Previously exceptions were buffered until all songs were loaded, now they are logged instantly (non-fatal warnings were already logged instantly).
- Demoted songparser log message levels to avoid excessive spam and to make better use of the new levels.
- Writing to std::clog is not thread-safe, causing occasional errors about invalid log prefixes (but I haven't seen crashes at least).
- Suggest replacing std::clog by a thread-local logger stream (global variable like std::clog), needs some work on logger backend.