[GHC] #8501: Improve error message when using rec/mdo keyword without RecursiveDo extention.

[GHC]

#8501: Improve error message when using rec/mdo keyword without RecursiveDo
extention.
-------------------------------------+-------------------------------------
        Reporter:  shelarcy          |                Owner:  ruperthorlick
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
  (Parser)                           |
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #3968             |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by ruperthorlick):

 * owner:  (none) => ruperthorlick
 * version:  7.6.3 => 8.1


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8501#comment:3>
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