Adjust error on bad enum

[som-snytt]

Fixes #22734

First cut is in refchecks, second cut is in checking.

Not sure if checking will induce cycles.

I have an old branch to improve "missing members" in refchecks, but too much work to leverage here.

This message may make more sense in checking, where it is enum-specific. The user must fix the enum and not the case (perhaps).

[som-snytt]

Not a cycle but maybe opaque means abstract. Anyway it should only check abstract term.

The other caution is that it's easier to let refchecks do the work. What enum syntax is legal? Every tweak to the test is an indentation problem. So it's easier to have confidence in the refcheck in the first commit, because the def $new signals that the anon class must be concrete.

-- Error: tests/run/option-extract.scala:2:5 ---------------------------------------------------------------------------
 2 |enum Option[+A]:
   |^
   |enum class Option has an abstract member and an unparameterized case
 3 |  case Some(x: A)
 4 |  case None
 5 |  opaque type ExtractResult[B] = (=> B) => B
 6 |  def extract[B](f: A => B): ExtractResult[B] =
 7 |    def result(default: => B): B = (this: Option[A]) match
 8 |      case None => default
 9 |      case Some(elem) => f(elem)
10 |    result
11 |  extension [B](er: ExtractResult[B])
12 |    def orElse(default: => B): B = er(default)
13 |end Option

[som-snytt]

This is the most important part of this PR.