Report server connection state

We must distinguish 3 cases for await_for_server():
 - an error occurred
 - no error occurred, the device is connected
 - no error occurred, the device is not connected (user requested to
   quit)

For this purpose, use an additional output parameter to indicate if the
device is connected (only set when no error occurs).

Refs #3085 <https://github.com/Genymobile/scrcpy/pull/3085>