[GHC] #10021: Add "Error:" prefix for compile-time error messages

[GHC]

#10021: Add "Error:" prefix for compile-time error messages
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        Reporter:  k-bx              |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.4
      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 k-bx):

 One more thing to consider. Current severities are:

 {{{
 data Severity
   = SevOutput
   | SevDump
   | SevInteractive
   | SevInfo
   | SevWarning
   | SevError
   | SevFatal
 }}}

 For severity `SevFatal`, would it make sense to output `fatal: <msg>`,
 stick to `error: <msg>`, or just keep it as it is right now?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10021#comment:10>
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