Handle missing or invalid broker server name/address and port gracefully.

Previously, X2Go Client would just literally crash if executed in broker mode and the broker didn't specify a server name/address (at all).

Handle this by checking if the broker has provided a server name/address and error out gracefully if it doesn't.

Additionally, try to parse the port value and check if it's in the valid [1,65535] range - error out gracefully if it's not.