Fix skins

I am really sorry, I did not catch those two errors during my final tests of the theme MR. The first one is that loading errors were actually throwing a JS error when trying to check if the theme that failed to load was the default one…

The second is that Chrome won't always fire an onload/onerror event on the link tag (it does the first time). I don't know why, I'll keep digging but meanwhile the only solution I found was to implement a fallback timer : if after 30 seconds the component has been updated, nothing has been fired, we check for it manually.