Better error message for missing Rank2Types (or RankNTypes) flag

For a rank-2 type signature when the Rank2Types language option isn't enabled, GHC gives:

    Illegal polymorphic or qualified type: (Data a) => a -> RnM a
    In the type signature for `rnVarsIn':
      rnVarsIn :: ((Data a) => a -> RnM a)
                  -> [Vdef] -> Exp -> S.Set Var -> RnM ([Vdef], Exp)

It would be good to suggest using -XRank2Types or -XRankNTypes. (Sorry if this is already fixed; I don't have a working HEAD to test it with.)

Trac metadata

Trac field	Value
Version	6.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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Operating system	Unknown
Architecture	Unknown