Stability fixes for back/retry re genseqs/useprompt

* Back and Retry buttons no longer pop a story chunk while in the
  "Select sequence to keep" menu
* No longer freezes if you retry with no story chunks beyond the initial
  prompt chunk
* When "Always Add Prompt" is on, allow Retry even if the prompt is the
  only chunk in the story
* Added error messages for Back and Retry buttons

static/application.js

@@ -1070,11 +1070,13 @@ $(document).ready(function(){
 	});
 	
 	button_actretry.on("click", function(ev) {
+		hideMessage();
 		socket.send({'cmd': 'retry', 'data': ''});
 		hidegenseqs();
 	});
 	
 	button_actback.on("click", function(ev) {
+		hideMessage();
 		socket.send({'cmd': 'back', 'data': ''});
 		hidegenseqs();
 	});