Add ParsableArguments#_errorPrefix to allow a more-customized error prefix than ParsableArguments#_errorLabel

[rgoldberg]

Add ParsableArguments#_errorPrefix to allow a more-customized error prefix than ParsableArguments#_errorLabel.

_errorLabel wasn't tested or documented outside DocC, so I didn't do anything more for _errorPrefix.

Resolve #836.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rgoldberg]

Rebased this on current main.

Thanks for merging #808. That will be very useful for my (and many other people's) completions.

[rauhul]

Rebased this on current main.

Thanks for merging #808. That will be very useful for my (and many other people's) completions.

How does this help completions?

[rgoldberg]

@rauhul This (#837) doesn't help completions. #808, which was just merged, helps completions.

I just wanted to thank you guys for merging #808.

I included the thanks in my prior comment on this PR (about having rebased this PR) since I was already writing a comment.

Sorry for any confusion.

[natecook1000]

Someday we can look at how to customize errors/help more completely, but this is a fine change for now.

[rgoldberg]

@natecook1000 Thanks for the merges.