main-afb-client-demo: Exit with a significant code

The program afb-client-demo now exits whith one of the
following code:

 0 - everything went well, no error
 1 - last received answer was an error
 2 - the remote hung up the connection
 3 - stdin either has too long line (16384) or has error
 4 - bad argument of the command line
 5 - cant connect to remote

Bug-AGL: SPEC-1619

Change-Id: Ia979c9ddcdae496ce205882b895cd9742f93f365
Signed-off-by: José Bollo <jose.bollo@iot.bzh>