Proxy error handling

[sjohnsonaz]

I added a small bit of error handling to the proxy section. It causes the server to return a 503 status, rather than exit.

[apixandru]

I'm having this exact same problem now, the server just crashes with regular chrome usage.
If the http server proxies /request1 to http://otherserver/request and i go and access /request1 and refresh a couple of times without letting the request go through, the http-server crashes because the proxy crashes.

[rafaelsales]

It feels like the owner has to add more maintainers to the project so that it scales, or it will become a ghost town

[BigBlueHat]

This generally looks good. It needs some tests and the whitespace changes removed. Once those are added, this seems like a good addition. Thanks!

[thornjad]

Closing in favor of the non-crashing logging in #477