snapcraft does not properly surface an error message during the pip presence checking on the second and subsequent runs

[Dmitrii Shcherbakov]

I ran into https://bugs.launchpad.net/snapcraft/+bug/1864589

from distutils.util import strtobool
modulenotfounderror: no module named 'distutils.util'

However, while debugging this and retrying I only got this printed in the error message which was confusing because snapcraft expects this to fail:

Failed to run '/root/stage/usr/bin/python3 -m pip': Exited with code 1.

I managed to trace what's going on underneath and it looks like the real exception is not properly displayed but it is about the same issue as in lp: #1864589

A full PDB trace:
https://paste.ubuntu.com/p/ykfm7TgVjm/

The real exception that gets raised:
> /snap/snapcraft/x1/lib/python3.6/site-packages/snapcraft/plugins/_python/_pip.py(231)_is_pip_installed()
-> raise e
(Pdb) output.lower()
'traceback (most recent call last):\n file "/root/stage/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main\n "__main__", mod_spec)\n file "/root/stage
/usr/lib/python3.6/runpy.py", line 85, in _run_code\n exec(code, run_globals)\n file "/root/parts/openstack-projects/install/lib/python3.6/site-packages/pip/__mai
n__.py", line 23, in <module>\n from pip._internal.cli.main import main as _main # isort:skip # noqa\n file "/root/parts/openstack-projects/install/lib/python3.6
/site-packages/pip/_internal/cli/main.py", line 10, in <module>\n from pip._internal.cli.autocompletion import autocomplete\n file "/root/parts/openstack-projects
/install/lib/python3.6/site-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>\n from pip._internal.cli.main_parser import create_main_parser\n fi
le "/root/parts/openstack-projects/install/lib/python3.6/site-packages/pip/_internal/cli/main_parser.py", line 7, in <module>\n from pip._internal.cli import cmdop
tions\n file "/root/parts/openstack-projects/install/lib/python3.6/site-packages/pip/_internal/cli/cmdoptions.py", line 19, in <module>\n from distutils.util impo
rt strtobool\nmodulenotfounderror: no module named \'distutils.util\''