Raise if file couldn't be read
Raise if file couldn't be read

File reading can lead to a `NoneType` value for `song` instead of
crashing.

Moreover, add debug info on file scanning, to detect which file fails.

nicolasmartinelli/oomusic,nicolasmartinelli/oomusic,DocMarty84/oomusic,DocMarty84/oomusic