extensionSystem: Unload stylesheet if extension is not loaded

We should not keep any reference to an extension custom stylesheet in case we
got an error while enabling that


(cherry picked from commit 3aea290adc)

js/ui/extensionSystem.js

@@ -136,6 +136,10 @@ function enableExtension(uuid) {
         _signals.emit('extension-state-changed', extension);
         return;
     } catch(e) {
+        if (extension.stylesheet) {
+            theme.unload_stylesheet(extension.stylesheet);
+            delete extension.stylesheet;
+        }
         logExtensionError(uuid, e);
         return;
     }