[GHC] #2851: Improve error message for failed deriving

[GHC]

#2851: Improve error message for failed deriving
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    Reporter:  guest                    |        Owner:                  
        Type:  feature request          |       Status:  reopened        
    Priority:  normal                   |    Milestone:                  
   Component:  Compiler (Type checker)  |      Version:  6.10.1          
    Severity:  minor                    |   Resolution:                  
    Keywords:                           |   Difficulty:  Unknown         
    Testcase:                           |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple         |  
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Comment (by guest):

 It would be an improvement if the error message just mentions standalone
 deriving, without giving the actual needed line of code.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2851#comment:6>
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