pgsql: Fix a few bogus statement type names in plpgsql error messages.

Fix a few bogus statement type names in plpgsql error messages.

plpgsql's error location context messages ("PL/pgSQL function fn-name line
line-no at stmt-type") would misreport a CONTINUE statement as being an
EXIT, and misreport a MOVE statement as being a FETCH.  These are clear
bugs that have been there a long time, so back-patch to all supported
branches.

In addition, in 9.5 and HEAD, change the description of EXECUTE from
"EXECUTE statement" to just plain EXECUTE; there seems no good reason why
this statement type should be described differently from others that have
a well-defined head keyword.  And distinguish GET STACKED DIAGNOSTICS from
plain GET DIAGNOSTICS.  These are a bit more of a judgment call, and also
affect existing regression-test outputs, so I did not back-patch into
stable branches.

Pavel Stehule and Tom Lane

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/4c3754ffe0f5f85cdecd01d7c1ab55df94559302

Modified Files
--------------
src/pl/plpgsql/src/pl_funcs.c               |    9 +++----
src/test/regress/expected/event_trigger.out |   34 +++++++++++++--------------
src/test/regress/expected/plpgsql.out       |   12 +++++-----
src/test/regress/expected/triggers.out      |   10 ++++----
4 files changed, 33 insertions(+), 32 deletions(-)