Don't use Play.current anywhere in Assets

Instead use Play.maybeApplication. This ensures that at very least templates using assets reverse routes can be rendered without a running application.
Hmm... I didn't notice that...

Actually there is a bug in there where resource not found is reported as an error too.  I don't think it's a big deal, this is only invoked by the assets action, not be the reverse router.  I don't think there's ever a reason why an end user will invoke the assets action outside of a running application.