Adding error handling to user file handling

[bgoesswe]

Error handling to user file upload / download / deletion should be improved by adding exceptions, and more relevant return values than true or false.

[jdries]

Note that user file handling is mainly unimplemented, also in backends. So all of the api will probably needs revisioning.

[soxofaan]

Also, since b4f2def and ee50d04, errors returned by the API (status > 400) are automatically converted to python exceptions by default now, so I don't think it is useful to keep this issue open