[securedrop-proxy] No error handling for subprocess.run calls

[legoktm]

None of the subprocess.run calls have any error handling. Either check=True should be passed or the return code should be explicitly checked.

>>> subprocess.run(['false'])
CompletedProcess(args=['false'], returncode=1)
>>> subprocess.run(['false'], check=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['false']' returned non-zero exit status 1.

It's also unclear to me why we shell out to cp instead of using shutil.copy2().