Fail on page-load failure (non-2xx result code), unless --ignore-failure

In most cases, you don't want to snapshot an error page (500, 502, etc).
This lets you choose whether or not you want to stop when these failures
occur (default), or go ahead and snapshot anyway.

Signed-off-by: Evan Owen <evan@cotap.com>
Merge pull request #1 from cotap/fix/load-failure

Fail on page-load failure (non-2xx result code), unless --allow-failure