[GHC] #10529: hpc: Improve error messages in readMix

[GHC]

#10529: hpc: Improve error messages in readMix
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              Reporter:  nh2         |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Code        |           Version:  7.10.1
  Coverage                           |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 See https://hackage.haskell.org/package/hpc-0.6.0.2/docs/src/Trace-Hpc-
 Mix.html#readMix

 The generic `error $ "can not find " ++ modName ++ " in " ++ show
 dirNames` error message is used for all kinds of cases, for example when
 `h == tixModuleHash tix` isn't true.

 It would be very helpful if the error message could be specific for these
 cases (e.g. "hashes don't agree") instead of being a catch-all for all
 `Nothing`s.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10529>
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