Parser: use 'error' token in error reporting rules

Summary:
It exempts us from 11 reduce/reduce conflicts and
12 shift/reduce conflicts.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Reviewers: simonpj, mikeizbicki, austin, simonmar

Reviewed By: simonmar

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D571