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.
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.