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