Improve error reporting for non-rigid GADT matches

Improve error reporting for non-rigid GADT matches

Following suggestions from users, this patch improves the error message
when a GADT match needs a rigid type:

 tcfail172.hs:19:10:
     GADT pattern match in non-rigid context for `Nil'
-      Solution: add a type signature
+      Probable solution: add a type signature for `is_normal'
     In the pattern: Nil
     In the definition of `is_normal': is_normal Nil = True

Now GHC tries to tell you what to give a type signature *for*.
Thanks to Daniel Gorin and others for the suggestions.