Introduce CaseClassCannotExtendsEnum message to replace error string from Checking

[skvithalani]

I have attempted to create an error message for case class extending enums.

Also in Checking.scala I could see approx 18 error messages as strings. So are these good to be picked up and massaged similar to CaseClassCannotExtendEnum in this PR ?

[dottybot]

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[Blaisorblade]

That works (the CI failure is spurious, sorry, we're tracking that in #5021)!

I see you've found #1589. If you feel like it and manage to improve the errors that's even better! This error doesn't say which enum is being extended, tho we know how to find that (it's in some commits in #5017, I'm splitting them).

[skvithalani]

It sounds good to have more elaborate error message. I will try to make it more user friendly.

[Blaisorblade]

So the relevant part of #5017 is #5028.

[skvithalani]

Thanks. That will be a good reference. I will take a look.

[Blaisorblade]

Since the PR is already OK (and the failure is spurious), merging to avoid conflicts! Follow-ups welcome, tho first the Dotty team should decide between #5028/#5031 I guess.