Improve error message

Prelude> class (A, B)

<interactive>:16:8: error:
    Unexpected type ‘A’
    In the class declaration for ‘GHC.Classes.(%,%)’
    A class declaration should have form
      class GHC.Classes.(%,%) a b where ...

in 7.10.2 it gave the more sensible

ghci> class (A, B)

<interactive>:30:7:
    Malformed head of type or class declaration: (A, B)

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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