pywatchman: raise a structured exception for socket connection issues
pywatchman: raise a structured exception for socket connection issues

This will help us robustly test for this error in upcoming patches.

nodakai/watchman,wez/watchman,facebook/watchman,dhruvsinghal/watchman,kwlzn/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,dhruvsinghal/watchman,wez/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,kwlzn/watchman,wez/watchman,facebook/watchman,wez/watchman,dhruvsinghal/watchman,facebook/watchman,dhruvsinghal/watchman,facebook/watchman,facebook/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,dhruvsinghal/watchman,kwlzn/watchman,dhruvsinghal/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,kwlzn/watchman,kwlzn/watchman,facebook/watchman,wez/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,facebook/watchman,kwlzn/watchman,facebook/watchman,facebook/watchman