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
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=8178d9480aeedfa20157bb98e21b5f3b6eb8a957

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