Provide more descriptive error message for non-argument variable parsing failure.

[drewmccormack]

Relates to issue https://forums.swift.org/t/how-to-exclude-members-from-parsing/34325/9

The existing message was completely unhelpful, and even erroneous in the sense that the issue is not related to an argument at all, but to a non-argument. The new message is a stop-gap until the actual problem causing the error is resolved, but it at least gives the user actionable information and makes it clear where the issue lies.

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

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

[natecook1000]

@swift-ci Please test

[natecook1000]

Thanks, @drewmccormack! I can't find any other path that leads to that error, so this looks like a good message.

[natecook1000]

@swift-ci Please test