Beautify Git subprocess errors

[defreng]

The current Git subprocess errors in foxops are quite unreadable:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/foxops/external/git.py", line 24, in git_exec
    return await check_call("git", *args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/foxops/utils.py", line 35, in check_call
    raise CalledProcessError(
  foxops.utils.CalledProcessError: Command '['git', 'push', '--porcelain', '-u', 'origin', b'main']' returned non-zero exit status 1. with stdout 'b'To https://[redacted].git\n!\trefs/heads/main:refs/heads/main\t[rejected] (fetch first)\nDone\n'' and stderr 'b"error: failed to push some refs to 'https://[redacted].git'\nhint: Updates were rejected because the remote contains work that you do\nhint: not have locally. This is usually caused by another repository pushing\nhint: to the same ref. You may want to first integrate the remote changes\nhint: (e.g., 'git pull ...') before pushing again.\nhint: See the 'Note about fast-forwards' in 'git push --help' for details.\n"'

It would be great to render the errors beautifully as they would be when running Git directly in a terminal. Print stdout and stderr separately.