Analyse following RPM build log snippet. Describe contents accurately, without speculation or suggestions for resolution.            
Your analysis must be as concise as possible, while keeping relevant information intact.
Snippet: 
Traceback (most recent call last):
  File "/usr/bin/copr-sources-custom", line 115, in <module>
    run_cmd(mock + ['--chroot', 'su - {0} -c {1}'.format(user, cmd)])
    ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/bin/copr-sources-custom", line 64, in run_cmd
    return subprocess.check_call(command, **kwargs)
           ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
Traceback from execution of program written in Python. It indicates that a sub-process launched with specific arguments failed. The exception raised is meant to indicate that failure.