Summary shows artifact servers which are not in use

I just forgot to put my certs/keys in place before a build.

I saw:

```
[--:--:--][ ][ main:core activity ] WARNING Failed to initialize remote : Failed to load credentials file: /home/tristan/work/project/certs/artifacts.key
[--:--:--][ ][ main:core activity ] WARNING Failed to initialize remote :8980: Failed to load credentials file: /home/tristan/work/project/certs/artifacts.key
```

And then in the summary I had:

```
Artifact cache servers
* :8980
push: False type: storage instance: None
loaded from: command line
* :8982
push: False type: index instance: None
loaded from: command line
```

I think this is incorrect, not sure if we should just *omit* this data for caches which we were unable to connect to, or perhaps we should just say `connected: True` or `connected: False` for clarity.

Reproduce a build with the artifact certificates or keys missing and compare the warning with the artifact cache server summary. Trace the summary generation from that behavior, then determine whether failed servers should be omitted or explicitly marked as disconnected. Done means the summary no longer presents unavailable servers as usable and regression coverage verifies the chosen behavior.

Written by the indexing model from the issue text.