fix(core): handle failure to clone a repository

3 jobs for topic/default/_cube-doctor.yml in 1 minute and 23 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Test
failed black-check #462028

00:00:29

passed flake8 #462029

00:00:14

passed py3 #462030

00:01:22

 
Name Stage Failure
failed
black-check Test
black run-test: commands[0] | black --check .
would reformat cube_doctor/old.py
Oh no! 💥 💔 💥
1 file would be reformatted, 46 files would be left unchanged.
ERROR: InvocationError for command /builds/cubicweb/cube-doctor/.tox/black/bin/black --check . (exited with code 1)
___________________________________ summary ____________________________________
ERROR: black: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1