GUAC-928 Restore file upload functionality, improve error appearance, and add translated units and file upload sizes.

guacamole/src/main/webapp/app/notification/styles/notification.css

@@ -27,6 +27,10 @@
     color: black;
 }
 
+.notification.error {
+    background: #FDD;
+}
+
 .notification .body {
     margin: 0.5em;
 }