diff --git a/IPython/html/base/handlers.py b/IPython/html/base/handlers.py
index fa58c2b..09d1210 100644
--- a/IPython/html/base/handlers.py
+++ b/IPython/html/base/handlers.py
@@ -370,7 +370,8 @@ def json_errors(method):
             message = e.log_message
             self.log.warn(message)
             self.set_status(e.status_code)
-            self.finish(json.dumps(dict(message=message)))
+            reply = dict(message=message, reason=e.reason)
+            self.finish(json.dumps(reply))
         except Exception:
             self.log.error("Unhandled error in API request", exc_info=True)
             status = 500
@@ -378,7 +379,7 @@ def json_errors(method):
             t, value, tb = sys.exc_info()
             self.set_status(status)
             tb_text = ''.join(traceback.format_exception(t, value, tb))
-            reply = dict(message=message, traceback=tb_text)
+            reply = dict(message=message, reason=None, traceback=tb_text)
             self.finish(json.dumps(reply))
         else:
             return result
diff --git a/IPython/html/services/contents/filemanager.py b/IPython/html/services/contents/filemanager.py
index 2ca1f41..6b0bb17 100644
--- a/IPython/html/services/contents/filemanager.py
+++ b/IPython/html/services/contents/filemanager.py
@@ -282,7 +282,7 @@ class FileContentsManager(ContentsManager):
                     model['content'] = bcontent.decode('utf8')
                 except UnicodeError as e:
                     if format == 'text':
-                        raise web.HTTPError(400, "%s is not UTF-8 encoded" % path)
+                        raise web.HTTPError(400, "%s is not UTF-8 encoded" % path, reason='bad format')
                 else:
                     model['format'] = 'text'
 
@@ -345,14 +345,14 @@ class FileContentsManager(ContentsManager):
         if os.path.isdir(os_path):
             if type_ not in (None, 'directory'):
                 raise web.HTTPError(400,
-                                u'%s is a directory, not a %s' % (path, type_))
+                                u'%s is a directory, not a %s' % (path, type_), reason='bad type')
             model = self._dir_model(path, content=content)
         elif type_ == 'notebook' or (type_ is None and path.endswith('.ipynb')):
             model = self._notebook_model(path, content=content)
         else:
             if type_ == 'directory':
                 raise web.HTTPError(400,
-                                u'%s is not a directory')
+                                u'%s is not a directory', reason='bad type')
             model = self._file_model(path, content=content, format=format)
         return model
 
diff --git a/IPython/html/static/edit/js/editor.js b/IPython/html/static/edit/js/editor.js
index 73b8bb7..ffbdb3a 100644
--- a/IPython/html/static/edit/js/editor.js
+++ b/IPython/html/static/edit/js/editor.js
@@ -79,6 +79,14 @@ function($,
                 that.events.trigger("file_loaded.Editor", model);
             },
             function(error) {
+                that.events.trigger("file_load_failed.Editor", error);
+                if (error.xhr.responseJSON.reason === 'bad format') {
+                    window.location = utils.url_path_join(
+                        that.base_url,
+                        'files',
+                        that.file_path
+                    );
+                }
                 cm.setValue("Error! " + error.message +
                                 "\nSaving disabled.");
                 that.save_enabled = false;
diff --git a/IPython/html/static/edit/js/main.js b/IPython/html/static/edit/js/main.js
index dc92426..2ff2845 100644
--- a/IPython/html/static/edit/js/main.js
+++ b/IPython/html/static/edit/js/main.js
@@ -73,7 +73,7 @@ require([
     page.show();
 
     window.onbeforeunload = function () {
-        if (!editor.codemirror.isClean(editor.generation)) {
+        if (editor.save_enabled && !editor.codemirror.isClean(editor.generation)) {
             return "Unsaved changes will be lost. Close anyway?";
         }
     };