Error message for failed openURL call

This will return an error message when a call to a URL such as twitter://post?message=hello%20world fails because the appropriate app to handle the scheme is not available.
Wrapped up hpub unpackaging into a block executed on a background thread
Merge commit '1e81513bddf7b3660e8e7b252cb819be1d7e39dd' into newsstand_notification