rocket::ignite does not fail if its desired port is in use

[ndarilek]

Rocket/codegen version: 0.2.4
Rust: rustc 1.18.0-nightly (bbdaad0dc 2017-04-14)

If you have one Rocket server running on port 8000, then launch another, the second claims to work even though it can't possibly since port 8000 is already in use.

I just hit this running one Rocket example in one terminal then forgetting about it. I then launched the Hello World example, which claimed to be running, and couldn't figure out why it kept 404ing on me. It wasn't until I found and killed the other server that I realized what was up.

[SergioBenitez]

This is a duplicate of #209. Please refer to that issue for details on the matter.