Added a CSS class to error messages.

Added a CSS class to error messages output by dash on load to allow them to be styled.

A workaround for this is to use a (fragile) selector path to match the element, e.g.

    #react-entry-point > div > div:nth-child(2):not([id]):not([class])

Author: ngnpope

src/APIController.react.js

@@ -83,15 +83,15 @@ class UnconnectedContainer extends Component {
         if (layoutRequest.status &&
             !contains(layoutRequest.status, [200, 'loading'])
         ) {
-            return (<div>{'Error loading layout'}</div>);
+            return (<div className="_dash-error">{'Error loading layout'}</div>);
         }
 
 
         else if (
             dependenciesRequest.status &&
             !contains(dependenciesRequest.status, [200, 'loading'])
         ) {
-            return (<div>{'Error loading dependencies'}</div>);
+            return (<div className="_dash-error">{'Error loading dependencies'}</div>);
         }