fix #10053: FieldError exception messages now show discriminant + type

[timotheecour]

• fix Better FieldError exception messages #10053

type Kind = enum k1, k2

type Foo = object
  case kind: Kind
  of k1:
    f1: int
  of k2:
    f2: int

proc main()=
  var foo = Foo()
  echo foo.f2
main()

before PR:
Error: unhandled exception: f2 [FieldError]

in most cases (eg see error msg i got from #11947) this is not very helpful

after PR:
Error: unhandled exception: 'f2' is not accessible using discriminant 'kind' of type 'Foo' [FieldError]

works with nim js and nim c

note

I have another PR to fix the VM case, but it depends on this PR so i'll wait until this is merged