avaudiocapture: Inform user when microphone access is not granted

macOS only asks once when requesting ressource access. If the user
declines for some reason, VLC will error out silently in subsequent
tries. Lets inform the user and explain better what he needs to
change in order to gain access again.

modules/access/avaudiocapture.m

@@ -233,6 +233,10 @@ static int Open(vlc_object_t *p_this)
             dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
             if (!accessGranted) {
                 msg_Err(p_demux, "Can't use the audio device as access has not been granted by the user");
+                vlc_dialog_display_error(p_demux, _("Problem accessing a system resource"),
+                    _("Please open \"System Preferences\" -> \"Security & Privacy\" "
+                      "and allow VLC to access your microphone."));
+
                 return VLC_EGENERIC;
             }
         }