[GHC] #9518: Improve error message for unacceptable role annotations

[GHC]

#9518: Improve error message for unacceptable role annotations
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              Reporter:  dmcclean    |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  low         |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Can you give a concrete example?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9518#comment:1>
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