[GHC] #7859: runghc should give an error when the path to ghc is invalid

[GHC]

#7859: runghc should give an error when the path to ghc is invalid
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Reporter:  monoidal                           |          Owner:                  
    Type:  bug                                |         Status:  new             
Priority:  normal                             |      Component:  None            
 Version:  7.6.3                              |       Keywords:                  
      Os:  Unknown/Multiple                   |   Architecture:  Unknown/Multiple
 Failure:  Incorrect warning at compile-time  |      Blockedby:                  
Blocking:                                     |        Related:                  
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 runghc has a -f option which allows to specify the path to GHC. However,
 it uses `rawSystem` and if the path is incorrect, we only get a nonzero
 exit code. For example, attempting to use "ghc -fdefer-type-errors X.hs"
 gives absolutely no output.

 Attached patch uses `findExecutable` and gives an error message if the
 executable is not found. This is not the ideal solution, but I think it's
 better than the current state.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7859>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler