Parser: revert some error messages to what they were before 7.10

Among doing other things, Phab:D201 (bc2289e13d9586be087bd8136943dc35a0130c88)
tried to improve the error messages thrown by the parser. For example a missing
else clause now prints "parse error in if statement: else clause empty" instead
of "parse error (possibly incorrect indentation or mismatched brackets)".

Some error messages got much worse however (see tests), and the result seems to
be a net negative. Although not entirely satisfactory, this commits therefore
reverts those parser changes.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1309

GHC Trac Issues: #10498

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