Split multi-branch unreachability warnings

Currently, if multiple branches in a case expression are unreachable, a single warning message is emitted:

module MultiBranch where

test x = case x of
    1 -> 2
    _ -> 3
    _ -> 4
    _ -> 5
    _ -> 6

..\curry-playground\MultiBranch.curry, line 4.5: Warning:
    Pattern matches are potentially unreachable
    In a case alternative:
      _
      _
      _ -> ...

It would be nice, however, to split this up into multiple warning messages to provide accurate line information for each branch:

..\curry-playground\MultiBranch.curry, line 6.5: Warning:
    Pattern match is potentially unreachable
    In a case alternative:
      _ -> ...

..\curry-playground\MultiBranch.curry, line 7.5: Warning:
    Pattern match is potentially unreachable
    In a case alternative:
      _ -> ...

..\curry-playground\MultiBranch.curry, line 8.5: Warning:
    Pattern match is potentially unreachable
    In a case alternative:
      _ -> ...