pgsql: Fix DROP INDEX CONCURRENTLY IF EXISTS.

Fix DROP INDEX CONCURRENTLY IF EXISTS.

This threw ERROR, not the expected NOTICE, if the index didn't exist.
The bug was actually visible in not-as-expected regression test output,
so somebody wasn't paying too close attention in commit
8cb53654dbdb4c386369eb988062d0bbb6de725e.
Per report from Brendan Byrd.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a6f43529ea6c3abd9207cea140ab231ac078746a

Modified Files
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src/backend/parser/gram.y                  |    2 +-
src/test/regress/expected/create_index.out |    2 +-
2 files changed, 2 insertions(+), 2 deletions(-)