Provide an explanatory message when attempted command parsing produces an empty result

[jayaddison]

This attempts to clarify the error messaging when the install_command entry in a tox environment configuration is empty.

• ran the linter to address style issues (tox -e fix)
• wrote pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

Continues-from #2715.

Relates to #2695, #2798.

[gaborbernat]

Please allow pushes to this PR.

[jayaddison]

Maintainer edits should be allowed @gaborbernat (looks like you were able to push some commits, let me know if I can help)

[gaborbernat]

ERROR: Permission to openculinary/tox.git denied to gaborbernat.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

seems not

[gaborbernat]

Why there's like 30 commits here?

[jayaddison]

Sometimes (often, recently) I'm a bit tedious about trying to keep the entire commit history for a pull request after it's open (even if it means including reverts, rather than squash committing).

It's kinda about being open about errors and mistakes during development. But yep, I do worry that it's annoying to read/review, and wonder how to figure out the balance (per-project/community).

I can reduce the commits down if that'd be useful (to a single commit, or to a few logical grouped changes).

[gaborbernat]

Replaced with #2807

[gaborbernat]

Also, please never use merge to make the branch up to date. I always prefer to rebase onto the main, which makes it easier to read and follow.

[jayaddison]

Ok, understood 👍 I'll do my best to remember not to do that again here.