Don't retry a mirror which failed on the first attempt

If the mirror attempt failed and we were able to report an error to the
user, it makes no sense to attempt a retry.  We don't have a way to
abort a mirror attempt yet, so if the user got a setting wrong and it's
failing for that reason, the only recourse they'd have would be to
restart the server.

tests/acceptance/flexnbd/fake_dest.rb

@@ -144,8 +144,7 @@ module FlexNBD
           client_sock = @sock.accept
         end
       rescue Timeout::Error
-        $stderr.puts err_msg
-        exit 1
+        raise Timeout::Error.new(err_msg)
       end
 
       client_sock