Bug 1105237 - [Loop] Move 'share' logic to a separate handler r=crdlc

I think that we should use here our error UI because alert has different look&feel than Loop. If you address it please add to your code this changes in https://github.com/mozilla-b2g/firefoxos-loop-client/pull/319/files#diff-49f57221c6d5bbc681a44862affc6e55L58 becasue the error screen does submit and the app is reloaded completely