Change SQLSTATE for event triggers "wrong context" message

When certain event-trigger-only functions are called when not in the
wrong context, they were reporting the "feature not supported" SQLSTATE,
which is somewhat misleading.  Create a new custom error code for such
uses instead.

Not backpatched since it may be seen as an undesirable behavioral
change.

Author: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAB7nPqQ-5NAkHQHh_NOm7FPep37NCiLKwPoJ2Yxb8TDoGgbYYA@mail.gmail.com

src/backend/commands/event_trigger.c

@@ -1416,7 +1416,7 @@ pg_event_trigger_dropped_objects(PG_FUNCTION_ARGS)
 	if (!currentEventTriggerState ||
 		!currentEventTriggerState->in_sql_drop)
 		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				(errcode(ERRCODE_E_R_I_E_EVENT_TRIGGER_PROTOCOL_VIOLATED),
 		 errmsg("%s can only be called in a sql_drop event trigger function",
 				"pg_event_trigger_dropped_objects()")));
 
@@ -1536,7 +1536,7 @@ pg_event_trigger_table_rewrite_oid(PG_FUNCTION_ARGS)
 	if (!currentEventTriggerState ||
 		currentEventTriggerState->table_rewrite_oid == InvalidOid)
 		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				(errcode(ERRCODE_E_R_I_E_EVENT_TRIGGER_PROTOCOL_VIOLATED),
 		 errmsg("%s can only be called in a table_rewrite event trigger function",
 				"pg_event_trigger_table_rewrite_oid()")));
 
@@ -1557,7 +1557,7 @@ pg_event_trigger_table_rewrite_reason(PG_FUNCTION_ARGS)
 	if (!currentEventTriggerState ||
 		currentEventTriggerState->table_rewrite_reason == 0)
 		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				(errcode(ERRCODE_E_R_I_E_EVENT_TRIGGER_PROTOCOL_VIOLATED),
 		 errmsg("%s can only be called in a table_rewrite event trigger function",
 				"pg_event_trigger_table_rewrite_reason()")));
 

src/backend/utils/errcodes.txt

@@ -278,6 +278,7 @@ Section: Class 39 - External Routine Invocation Exception
 39004    E    ERRCODE_E_R_I_E_NULL_VALUE_NOT_ALLOWED                         null_value_not_allowed
 39P01    E    ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED                      trigger_protocol_violated
 39P02    E    ERRCODE_E_R_I_E_SRF_PROTOCOL_VIOLATED                          srf_protocol_violated
+39P03    E    ERRCODE_E_R_I_E_EVENT_TRIGGER_PROTOCOL_VIOLATED                event_trigger_protocol_violated
 
 Section: Class 3B - Savepoint Exception