pgsql: Fix a number of places that produced XX000 errors in the regress

Fix a number of places that produced XX000 errors in the regression tests.

It's against project policy to use elog() for user-facing errors, or to
omit an errcode() selection for errors that aren't supposed to be "can't
happen" cases. Fix all the violations of this policy that result in
ERRCODE_INTERNAL_ERROR log entries during the standard regression tests,
as errors that can reliably be triggered from SQL surely should be
considered user-facing.

I also looked through all the files touched by this commit and fixed
other nearby problems of the same ilk. I do not claim to have fixed
all violations of the policy, just the ones in these files.

In a few places I also changed existing ERRCODE choices that didn't
seem particularly appropriate; mainly replacing ERRCODE_SYNTAX_ERROR
by something more specific.

Back-patch to 9.5, but no further; changing ERRCODE assignments in
stable branches doesn't seem like a good idea.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/09cecdf285ea9f51aed669f9ea1ba840197d49d0

Modified Files
--------------
contrib/tablefunc/tablefunc.c | 40 +++++++++++-------
src/backend/access/common/reloptions.c | 19 +++++----
src/backend/access/heap/heapam.c | 8 +++-
src/backend/commands/copy.c | 32 ++++++++++-----
src/backend/commands/vacuum.c | 5 ++-
src/backend/executor/execQual.c | 6 ++-
src/backend/utils/adt/txid.c | 13 ++++--
src/pl/plperl/plperl.c | 70 +++++++++++++++++++++-----------
src/pl/plpython/plpy_elog.c | 2 +-
src/pl/plpython/plpy_exec.c | 12 ++++--
src/pl/tcl/pltcl.c | 51 +++++++++++++++--------
src/test/regress/expected/txid.out | 10 ++---
src/test/regress/regress.c | 6 ++-
13 files changed, 181 insertions(+), 93 deletions(-)