[GHC] #16270: Report multiple errors during parsing

[GHC]

#16270: Report multiple errors during parsing
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
  (Parser)                           |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Parsing failures currently abort the parsing process (with
 `parseErrorSDoc`) even if it can continue and report multiple errors at
 once.

 This is easy to fix by creating a new combinator, `addError`, similar to
 `addWarning`, which would add the error message to the accumulator but
 continue parsing.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16270>
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