Remove the word "skolem" from user error messages

Skolem variables are not, as best I can tell, part of the type system; they are part of the implementation of the type checking algorithm. As such, it seems inappropriate to mention them in error messages. Instead, the error messages should explain things at the Haskell level.

    Couldn't match expected type ‘t’ with actual type ‘a’
      because type variable ‘a’ would escape its scope
    This (rigid, skolem) type variable is bound by...

I'd much rather see an explanation using words like "existentially quantified" and "right-hand side of a pattern match".

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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