pgoutput: Raise an error for missing protocol version parameter.

Currently, we give a misleading error if the user omits to pass the
required parameter 'proto_version' in SQL API
pg_logical_slot_get_changes() or during START_REPLICATION protocol
message. The error raised is as follows which indicates that the wrong
version is passed.
ERROR:  client sent proto_version=0 but server only supports protocol 1 or higher

Author: Emre Hasegeli
Reviewed-by: Peter Smith, Amit Kapila
Discussion: https://postgr.es/m/CAE2gYzwdwtUbs-tPSV-QBwgTubiyGD2ZGsSnAVsDfAGGLDrGOA@mail.gmail.com

src/backend/replication/pgoutput/pgoutput.c

@@ -400,6 +400,16 @@ parse_output_parameters(List *options, PGOutputData *data)
 		else
 			elog(ERROR, "unrecognized pgoutput option: %s", defel->defname);
 	}
+
+	/* Check required options */
+	if (!protocol_version_given)
+		ereport(ERROR,
+				errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				errmsg("proto_version option missing"));
+	if (!publication_names_given)
+		ereport(ERROR,
+				errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				errmsg("publication_names option missing"));
 }
 
 /*
@@ -449,11 +459,6 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 					 errmsg("client sent proto_version=%d but server only supports protocol %d or higher",
 							data->protocol_version, LOGICALREP_PROTO_MIN_VERSION_NUM)));
 
-		if (data->publication_names == NIL)
-			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("publication_names parameter missing")));
-
 		/*
 		 * Decide whether to enable streaming. It is disabled by default, in
 		 * which case we just update the flag in decoding context. Otherwise