pgsql: Throw an appropriate error if ALTER COLUMN TYPE finds a dependen

Throw an appropriate error if ALTER COLUMN TYPE finds a dependent trigger.

Actually making this case work, if the column is used in the trigger's
WHEN condition, will take some new code that probably isn't appropriate
to back-patch. For now, just throw a FEATURE_NOT_SUPPORTED error rather
than allowing control to reach the "unexpected object" case. Per bug #5688
from Daniel Grace. Back-patch to 9.0 where the possibility of such a
dependency was introduced.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=1f0b62e8c28026f5d1d3693087f7b4aef6a191af

Modified Files
--------------
src/backend/commands/tablecmds.c | 19 ++++++++++++++++++-
1 files changed, 18 insertions(+), 1 deletions(-)