If the remote of a NuGet v3 proxy is not reachable it breaks requests to a NuGet group

Configure a NuGet group repository that contains several hosted repositories, and also a proxy repository configured for a v3 API.  Ensure that the remote of the proxy repository cannot be reached on the network.

Send a request to /v3/index.json to the group repository. This will fail with a 502 error code.

Expected:  Failure to reach the remote of one proxy repository in a group should not cause the entire request to fail.  We should log the failure, process the other repositories in the group, and return their content.