prserv/serv: Improve error message when prserver cannot bind to supplied host address

If localhost resolves to a remote address (due to a misconfigured network),
starting the pr server will fail without useful information.

To reproduce, add '<bogus ip> localhost' to /etc/hosts and run
'bitbake -p'. The error message will be:

ERROR: Timeout while attempting to communicate with bitbake server
ERROR: Could not connect to server False:

Running 'bitbake-prserv --host=localhost --port=0 --start' will fail with:

error: [Errno 99] Cannot assign requested address

Since these errors does not show the IP address of the attempted socket
binding, this results in a lot of wasted time looking at firewall rules, etc.

This patch results in the following error message if the socket binding fails:

PR Server unable to bind to <bogus ip>:0

Signed-off-by: Konrad Scherer <Konrad.Scherer@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>