npm proxy receiving connection reset responds to client with status 500 instead of 404

If an npm proxy repository receives a connection reset when connecting to it's remote a 500 response is returned by Nexus. This is true even if the inbound request is made through a group repository. Also, the proxy is not auto-blocked when this happens.

Moving the affected proxy to the bottom of an npm group repository does not provide a workaround, inbound metadata queries result in outbound queries to all members of the group.

Expected:

1. The proxy should not return a 500 response when the remote can't be reached, it should return 404, and log the connection problem.
2. The proxy should auto-block when it's remote can't be reached.

Testing note... I used "cynic" to set this up, so I could quickly get the "connection reset" error happening. Note that "cynic" does not run on Mac OSX, so it's necessary to use Linux if you want to use this method.

https://pypi.python.org/pypi/cynic/1.0