Playing should stop when an error occurs

When an error occurs during playback, exaile (at least with the normal engine) will just play the next track. In addition an error dialog will pop up. This seems reasonable at the first look, until something like the following happens:
- you have your mp3 files on a separate volume
- the volume is not mounted
- you start a playlist
The result is that every unavailable track is tried to play and you end up with a *lot* of dialogs you have to click away. Since the dialogs are modal, you cannot stop the playing and you even have to click the dialogs away in the 'right' order. Practically this means you have to kill -9 exaile if the playlist contains more than a few 10 tracks.

Since the dialog is modal anyway, I propose to stop playing in case of an error. The attached patch just removes the call to the 'eof_func' in case of an error, which is a bit hacky, but does the job (for the normal engine).

The exaile version used is trunk.