Arrange to print the relevant key values when reporting a foreign-key

violation.  Also, factor out some duplicate code in the RI triggers.
Patch by Dmitry Tkach, reviewed by Stephan Szabo and Tom Lane.

src/test/regress/expected/alter_table.out

@@ -326,7 +326,7 @@ ERROR:  ALTER TABLE: column "b" referenced in foreign key constraint does not ex
 -- Try (and fail) to add constraint due to invalid data
 ALTER TABLE tmp3 add constraint tmpconstr foreign key (a) references tmp2 match full;
 NOTICE:  ALTER TABLE will create implicit trigger(s) for FOREIGN KEY check(s)
-ERROR:  tmpconstr referential integrity violation - key referenced from tmp3 not found in tmp2
+ERROR:  tmpconstr referential integrity violation - key (a)=(5) referenced from tmp3 not found in tmp2
 -- Delete failing row
 DELETE FROM tmp3 where a=5;
 -- Try (and succeed)

src/test/regress/expected/cluster.out

@@ -248,7 +248,7 @@ SELECT a,b,c,substring(d for 30), length(d) from clstr_tst;
 
 -- Verify that foreign key link still works
 INSERT INTO clstr_tst (b, c) VALUES (1111, 'this should fail');
-ERROR:  clstr_tst_con referential integrity violation - key referenced from clstr_tst not found in clstr_tst_s
+ERROR:  clstr_tst_con referential integrity violation - key (b)=(1111) referenced from clstr_tst not found in clstr_tst_s
 SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
     conname     
 ----------------