Fail if the first hop is not in the consensus.

Instead of just relying on Stem's error message, print a human-readable error
message and exit if the given first hop cannot be found in the consensus.
Increase timeout when bootstrapping Tor.

On slow network links, it can take Tor quite a while to download the consensus.
As a result, the timeout is increased from 30 to 90 seconds.
Merge branch 'master' of github.com:NullHypothesis/exitmap

Conflicts:
	exitmap.py