Updated the default rescue response (i.e when exception or bad internal error occurs).

Factorized the implementation in WGI_RESCUE_EXECUTION, and now by redefining the `WGI_EXECUTION.execute_rescue (...)` procedure, it is possible to have a custom response on such rescued execution.

library/server/ewsgi/specification/connector/wgi_execution.e

@@ -42,6 +42,14 @@ feature -- Execution
 			is_valid_end_of_execution: is_valid_end_of_execution
 		end
 
+feature {WGI_EXPORTER, WGI_CONNECTOR} -- Execution: rescue
+
+	execute_rescue (e: detachable EXCEPTION)
+			-- Execute on rescue.
+		do
+			(create {WGI_RESCUE_EXECUTION}).execute (request, response, e)
+		end
+
 feature -- Status report
 
 	is_valid_end_of_execution: BOOLEAN