Handle errors in different phases differently (#83)

This patch clarifies the different phases that happen while servicing a
request.  A network error occurs during one of those phases, and we now
handle errors differently depending on which phase they occur in:

  - `include_subdomains` policies can only be used to generate reports
    about DNS resolution errors, since the policy author can only
    confirm ownership of the DNS tree, and not of all of the servers
    that those domain names resolve to.

  - If the resolved IP address for an origin changes between when a
    policy is received, and when its used to generate a report, we don't
    report any details about the connection and application phases, and
    only report that the IP address changed.  This prevents DNS
    rebinding attacks.

Issue: #74

Author: dcreager