calc: improve help output, test coverage (now 100%)

[dgw]

Error cases weren't covered by the @plugin.example tests. Fixed that.

Also declared a few more examples that should appear in the help output, to showcase more of the plugin's capabilities.

Follow-up to https://github.com/sopel-irc/sopel/pull/2503#issuecomment-1676205118.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches

[dgw]

@Exirel @SnoopJ I had an itch to scratch in sopel/tools/calculation.py too. 😁

Edit: 09:57:57 <+xrl> dgw: it's still good for me

[dgw]

With apologies for the force-push, I had to rebase in order to make the wording change addressing @SnoopJ's stylistic concern over one error message. (Blame the merging of #2504.) Scout's honor that that was the only thing I changed, but this diff is small enough that you guys can definitely re-review it if you want between now and when it gets merged.