Make error message clearer when the namespace is incorrect

[maruth-stripe]

Updates the error message when the namespace is incorrect to specify the exact action the developer must take to fix the error.

The error message as it exists today does not immediately signal an action to fix it to developers unfamiliar with Rubocop terminology.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

I'm on the fence about the proposed message change, as I don't think the old one was unclear. @rubocop/rubocop-core any thoughts?

[bquorning]

I find the new text easier to understand.

Please squash commits though 😅

[koic]

There are pros and cons to both the before and after changes of the message. I don't have a strong opinion on this. It's left to discretion for this.

[maruth-stripe]

@bquorning squashed the commits!

@bbatsov Thanks for the feedback! While the old message may not be unclear, developers have told us they would find a message like the new one more clear as it tells them precisely what change to make. Hence, just trying to improve that experience for them :D

[bbatsov]

Fair enough. I'm OK with the proposed change.