Updated to use socket.gaierror as OSError was deprecated

A failed call to ``` socket.gethostbyname(local_hostname)``` now throws ```socket.gaierror``` if it's fails to get a host, not the now deprecated OSError. This caused it to not execute the error condition, making it impossible to enter a hostname manually. 

This fixes https://github.com/grr-hackers/grr/issues/4