bug#50918: "server is somewhat slow" error is more annoying than helpful

[raid5atemyhomework]

So for the past several months I have been getting errors like these:

```
guix substitute: warning: while fetching 
https://ci.guix.gnu.org/nar/lzip/k9wmrk5m91599lk8gd4rc7h4df642qw0-curl-7.74.0: 
server is somewhat slow
guix substitute: warning: try `--no-substitutes' if the problem persists
guix substitute: error: connect*: Connection timed out
substitution of /gnu/store/k9wmrk5m91599lk8gd4rc7h4df642qw0-curl-7.74.0 failed
```


These errors are more annoying than helpful to me.  There's nothing I can do 
about the servers, and the ISP I have is the best I can get with my budget (I 
already have two ISPs, and the one I use for Guix is the one that is more 
reliable, believe me).

At the least, there should be some way to downgrade this from a "connection 
timed out" error to just keep on keeping on.  Or at least to tell the tool to 
just keep retrying the download for some specified number of times before 
actually giving up (in practice 2 or 3 times is enough on my part of the 
Internet).

Thanks
raid5atemyhomework