cli: Don't crash when subcommands fail importing
This is an issue when swh.model is installed but not dulwich: swh.model.cli imports dulwich unconditionally, so /usr/bin/swh fails for all subcommands instead of just ignoring the issue.
Test Plan
python3 -m venv cli-fail
. ./cli-fail/bin/activate
pip install swh.core swh.model
./cli-fail/bin/swh
Doesn't fail with ModuleNotFoundError: No module named 'dulwich'
Migrated from D2225 (view on Phabricator)