[GHC] #639: Give civilised error messages when desugaring Template Haskell brackets

[GHC]

#639: Give civilised error messages when desugaring Template Haskell brackets
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    Reporter:  simonpj at microsoft.com  |        Owner:             
        Type:  task                   |       Status:  new        
    Priority:  normal                 |    Milestone:             
   Component:  Compiler               |      Version:  6.4.1      
    Severity:  normal                 |     Keywords:             
          Os:  Unknown                |   Difficulty:  Easy (1 hr)
Architecture:  Unknown                |  
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DsMeta fails with a panic if it encounters something it can't desugar into
 TH syntax.  This is bad: the error should be civilised.  But there's no
 current provision for desugarer *errors*, only *warnings*.

 Add errors, and perhaps promote the error/warning box into the part that
 is shared between all uses of IOEnv (renamer, typechecker, desugarer)

 Nothing hard, a bit tedious.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/639>
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