use numeric error code for detecting unique violations on sl_nodelock

The text of the error message has changed slightly in different postgresql
versions, ie 8.3+.  Using the numeric error code will protect us from this.
This fixes bug 276.

Conflicts:

	src/slon/local_listen.c

Branch
------
REL_2_1_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=a8ef5576a7ffdb7b8120f040d05ec4473a175839

Modified Files
--------------
RELEASE                 |    3 +++
src/slon/local_listen.c |    8 +++++---
2 files changed, 8 insertions(+), 3 deletions(-)