internal errors: raise a ValueError if user_data.json is corrupted
internal errors: cleanup in previous test (ValueError)
internal errors: server raises an OSError (and return 500) if finds an existing directory, while tries to create a new user
tests: import _API_PREFIX
tests: fixed obsolete tests
internal errors: removed get_user in server, because it can't raise any error and it's useless
internal errors: check IOError when an expected file is missing
internal errors: check if the service code 500 is really returned
internal errors: json needed by the missing file test
some style
internal errors: removed server_errors.py (now it's useless)
internal errors: fixed obsolete test