Adjust error on bad enum

[som-snytt]

Fixes #22734

[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.

[som-snytt]

Rebased.

[som-snytt]

Passes locally.

Test 'tests/run/i23245a' failed with output:                                    

Exception: java.lang.NullPointerException thrown from the UncaughtExceptionHandler in thread "main"

[som-snytt]

Dropped the check in Checking, where the idea was to error the enum if a case will error as abstract; the test was not correct and not worth the extra code. The message should be improved in refchecks instead.