proxy handler warn/error on fails; fallback to localhost option

[dale-wahl]

Noticed that setting up proxies could cause processors to fail without every raising an error (just looks like bad requests to the processors).

• Added health tracking to the proxy handler
• warns on proxy fail by proxy (in case issue is only with individual ones)
• re-queues request for other proxies
• if all proxies fail, optionally fallback to localhost/direct request (default is True since, well, that's the norm anyway)

In the event there is no fallback option and all proxies fail, we might consider raising something so that the processors known about this condition if we want to inform the user. But the admin should be informed now at least.