Linux/MIDI: show error messages, fix MIDI filenames

[dlichtenberger]

Hi,

here are two small patches against git master (1d13f923fae0eed4db9f7d4419804983d1a5f619) for running the sdl_mixer MIDI backend on Linux:

* music-tab-fix-filenames.patch - fixes the case of a few MIDI filenames for filesystems that are case sensitive
* sdl_mixer-show-midi-errors.patch - SDL returns quite useful error messages when something doesn't work (e.g. when the timidity++ software synthesizer is not installed). This patchs prints the error message obtained from SDL when a file could not be loaded. This also covers the case when the MIDI file does not exist.

Cheers,
Daniel

[prissi]

Incorporated, thanks!