crash when concat a string with a PosixPath

[timokeller]

DeepBach/DeepBach/data_utils.py

Line 265 in f069695

print('KeyError: File ' + chorale_file + ' skipped')

DeepBach/DeepBach/data_utils.py

Line 268 in f069695

'FloatingKeyException: File ' + chorale_file + ' skipped')

This should be str(chorale_file) because you can't concat a string with a PosixPath.