ftp: Check connection is usable when acquiring
ftp: Check connection is usable when acquiring

Qhen acquiring a cached connection, it might have timeout'ed or
otherwise not be available anymore. We don't want these connection
errors to propagate into the job we're handling and instead just acquire
the next connection.

https://bugzilla.gnome.org/show_bug.cgi?id=591054

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs