TH doesn't verify name types during conversion

Angerman reported that a use of the `deriveEq` splice is causing GHC to abort with an assertion failure:

zonkExpr env (HsVar x (L l id))
  = ASSERT2( isNothing (isDataConId_maybe id), ppr id )
    return (HsVar x (L l (zonkIdOcc env id)))

I suspect the deriveEq1 is calling varE with a DataCon name. We should catch this case and throw a better error message.