Fix DROP OPERATOR FAMILY IF EXISTS.

Essentially, the "IF EXISTS" portion was being ignored, and an error
thrown anyway if the opfamily did not exist.

I broke this in commit fd1843ff; so
backpatch to 9.1.X.

Report and diagnosis by KaiGai Kohei.

src/backend/commands/opclasscmds.c

@@ -1613,9 +1613,8 @@ RemoveOpFamily(RemoveOpFamilyStmt *stmt)
 	tuple = OpFamilyCacheLookup(amID, stmt->opfamilyname, stmt->missing_ok);
 	if (!HeapTupleIsValid(tuple))
 	{
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("operator family \"%s\" does not exist for access method \"%s\"",
+		ereport(NOTICE,
+				(errmsg("operator family \"%s\" does not exist for access method \"%s\", skipping",
 				   NameListToString(stmt->opfamilyname), stmt->amname)));
 		return;
 	}

src/test/regress/expected/drop_if_exists.out

@@ -214,7 +214,7 @@ ERROR:  access method "no_such_am" does not exist
 DROP OPERATOR FAMILY test_operator_family USING btree;
 ERROR:  operator family "test_operator_family" does not exist for access method "btree"
 DROP OPERATOR FAMILY IF EXISTS test_operator_family USING btree;
-ERROR:  operator family "test_operator_family" does not exist for access method "btree"
+NOTICE:  operator family "test_operator_family" does not exist for access method "btree", skipping
 DROP OPERATOR FAMILY test_operator_family USING no_such_am;
 ERROR:  access method "no_such_am" does not exist
 DROP OPERATOR FAMILY IF EXISTS test_operator_family USING no_such_am;