internal/wayland: improve error handling

Note: wl_registry_add_listener is undocumented everywhere. Its implementation calls wl_proxy_add_listener which returns 0 on success or -1 on failure.
Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/system/output.go

@@ -40,6 +40,10 @@ func (e *OpError) Error() string {
 }
 
 func (e *OpError) Message() string {
+	if m, ok := message.GetMessage(e.Err); ok {
+		return m
+	}
+
 	switch {
 	case e.Msg != "":
 		return e.Error()