FIX: properly handle errors when creating a topic

app/assets/javascripts/discourse/controllers/composer.js.es6

@@ -234,7 +234,6 @@ export default Ember.ObjectController.extend(Presence, {
       imageSizes: this.get('view').imageSizes(),
       editReason: this.get("editReason")
     }).then(function(result) {
-
       if (result.responseJson.action === "enqueued") {
         self.send('postWasEnqueued', result.responseJson);
         self.destroyDraft();

app/assets/javascripts/discourse/lib/ajax-error.js.es6

@@ -7,20 +7,33 @@ function extractError(error) {
     Ember.Logger.error(error);
   }
 
-  let parsedError;
+  if (error.jqXHR) {
-  if (error.responseText) {
+    error = error.jqXHR;
+  }
+
+  let parsedError, parsedJSON;
+
+  if (error.responseJSON) {
+    parsedJSON = error.responseJSON;
+  }
+
+  if (!parsedJSON && error.responseText) {
     try {
-      const parsedJSON = $.parseJSON(error.responseText);
+      parsedJSON = $.parseJSON(error.responseText);
-      if (parsedJSON.errors) {
-        parsedError = parsedJSON.errors[0];
-      } else if (parsedJSON.failed) {
-        parsedError = parsedJSON.message;
-      }
     } catch(ex) {
       // in case the JSON doesn't parse
       Ember.Logger.error(ex.stack);
     }
   }
+
+  if (parsedJSON) {
+    if (parsedJSON.errors && parsedJSON.errors.length > 0) {
+      parsedError = parsedJSON.errors[0];
+    } else if (parsedJSON.failed) {
+      parsedError = parsedJSON.message;
+    }
+  }
+
   return parsedError || I18n.t('generic_error');
 }
 
@@ -28,11 +41,10 @@ export function throwAjaxError(undoCallback) {
   return function(error) {
     // If we provided an `undo` callback
     if (undoCallback) { undoCallback(error); }
-
     throw extractError(error);
   };
 }
 
-export function popupAjaxError(err) {
+export function popupAjaxError(error) {
-  bootbox.alert(extractError(err));
+  bootbox.alert(extractError(error));
 }