Run Flashlight regardless of the result of port mapping closes #1725

This should do the trick @oxtoacart 
Well, it's not 100% clear to me under which conditions one will get an invalid port.  I could imagine the API returning a 0 if the router supports UPnP but it's turned off.  One case I know of for sure is that the router supports UPnP, but doesn't allow mapping low ports (e.g. 443).  That is the case with my router, which is why I ended up in this branch of the code to begin with.

I think we should probably just keep the message generic and always instruct people to manually map a port of we can't do it automatically for any reason.

Whatever we do, I don't think we can pass 0 here, because flashlight will fail to start with such a port.