BUG Redisplay CMS forms with validation errors

Guess the fragment based on the returned HTML.
The validation error HTTP response is generated by Form rather than
the controller (LeftAndMain), so we can't set custom PJAX headers easily.

admin/javascript/LeftAndMain.js

@@ -328,7 +328,7 @@ jQuery.noConflict();
 			 * Can be hooked into an ajax 'success' callback.
 			 */
 			handleAjaxResponse: function(data, status, xhr) {
-				var self = this, url, selectedTabs;
+				var self = this, url, selectedTabs, guessFragment;
 
 				// Pseudo-redirects via X-ControllerURL might return empty data, in which
 				// case we'll ignore the response
@@ -343,7 +343,9 @@ jQuery.noConflict();
 					newFragments = data;
 				} else {
 					// Fall back to replacing the content fragment if HTML is returned
-					newFragments['Content'] = data;
+					$data = $(data);
+					guessFragment = $data.is('form') ? 'CurrentForm' : 'Content';
+					newFragments[guessFragment] = $data;
 				}
 
 				// Replace each fragment individually