[transfer/open_uri] rescue StandardError on download fail

There are too many possible exceptions that can be raised while trying
to download a file, from OpenURI::HTTPError, OpenSSL::SSL::SSLError,
SocketError, to Errno::ENOSPC. Instead of playing exception bingo, we
rescue all descendants of StandardError, all of which should hopefully
be things we can handle _somewhat_ gracefully.
Merge branch 'openuri-rescue_standard_error'