[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:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Code Coverage     |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by nh2):

 Yes that looks great to me!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10529#comment:4>
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