Add error matching dispatch for exception type
* usage:  with error.expected(KeyError):   x = d['nonexistentkey']
Prevent tests hang when ip_echo is down.

It was here before, but when I was changing the fixture the last time, I introduced this back. py.test touches everything to find a fixture, so it was touching the lazycached host
Merge pull request #736 from mfalesni/prevent_touching_error

Prevent tests hang when ip_echo is down.