Make program exit gracefully when binding fails.

[richard-uk1]

Also give good error message when address is in use.

I've reopened this PR. If you want to test it before merging

git remote add derekdreery https://github.com/derekdreery/basic-http-server
git fetch derekdreery
git checkout derekdreery/handle_addr_in_use

(I hate looking up git commands so I thought I'd save you the trouble :P)

It should work - I'm not sure why it wasn't for you. Maybe it was to do with a merge artifact, which this shouldn't exhibit since it's rebased on master.