Lint the validity of help categories

[Niloth-p]

What does this PR do, and why?

Check for typos in category names.
Previously, these typos would go unnoticed, with the key binding going missing from the help menu.

Outstanding aspect(s)

• Feedback welcome on the formatting and phrasing of the error message printed
• This could be put on hold for the moment and be merged along with the major changes we will be doing to lint-hotkeys after the addition of contexts. But, that might take a while.

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[rsashank]

Thanks for working on this @Niloth-p!
The intention is clear. I've tested this on my PC and reviewed the code, and it LGTM 👍.

[zormit]

That seems like a nice idea to check for this and will hopefully improve the developer experience :) Thanks for working on this!

It's a bit adjacent to this PR, but while you're in this area of the code, I've left a comment to improve it. I think it wouldn't hurt to fix it as part of this PR, as it's a very small PR anyways.

I'm still approving it, because in my opinion it's optional to fix.

[zormit]

it's rather independent of this change, but it's quite strange to me to use 0 and 1 instead of a boolean here. I know sys.exit wants to have an integer, but it should in my opinion only be converted to it at the very last moment.

[neiljp]

@Niloth-p As discussed briefly via video, this would have been useful when I rebased a PR already :)

I left a few small notes and otherwise looks good to go. As per @zormit's notes, the refactoring isn't vital but would make it cleaner, particularly as we don't have tests for this.

[neiljp]

NIT: For a dev tool this isn't as important, but with this number of categories we already run quite far to the right, so a vertical list may be cleaner to read.

[neiljp]

While this likely works, an explicit conversion here would be good, as per #1518 (comment)

[neiljp]

NIT: As it's now a boolean, it'd also be useful to have a cleaner name for this.

Maybe is_any_error_present, or similar.