apns crashes with {case_clause,{error,{closed,"The connection was lost."}}

Hi,

Function apns_connection:push/4
https://github.com/inaka/apns4erl/blob/master/src/apns_connection.erl/#L476-L483
is missing several values returned by gun:await/3.

This will cause the function crashes, in my case, when gun returns {error,{closed,"The connection was lost."}}.

Can someone please fix this? at least I think it would be useful to inform clients about connection errors, beside timeout.