SI-7330 better error when pattern isn't a value

Somehow an applied type managed to sneak past the type checker in pattern mode.
Patterns must be values, though.

`case C[_] =>` was probably meant to be `case _: C[_] =>`
Advice is dispensed accordingly. (Generalizing the existing advice machinery.)
Merge pull request #2358 from adriaanm/ticket-7330

SI-7330 better error when pattern's not a value