Report failures to a remote server.

A "failure" is defined using response codes.  The following codes are
treated as failure codes:

- any 4xx that isn't 403 or 404
- any 5xx
- any 0 (this is a wget code for network errors, protocol errors, etc)
Accept wget invocations that had network and protocol errors.

We're recording these via the Lua script; therefore, we'll know which
URLs issued network/protocol errors and accepting the results is fine.
(We don't currently differentiate between the two when logging, but we
could if it ends up being useful.)
Merge pull request #3 from ArchiveTeam/report-failure

Report failures to remote collector