Show when we retried requests (#4725)
In #3514 and #2755, users had intermittent network errors, but it was
not always clear whether we had already retried these requests or not.
Building upon https://github.com/TrueLayer/reqwest-middleware/pull/159,
this PR adds the number of retries to the error message, so we can see
at first glance where we're missing retries and where we might need to
change retry settings.
Example error trace:
```
Could not connect, are you offline?
Caused by: Request failed after 3 retries
Caused by: error sending request for url (https://pypi.org/simple/uv/)
Caused by: client error (Connect)
Caused by: dns error: failed to lookup address information: Name or service not known
Caused by: failed to lookup address information: Name or service not known
```
This code is ugly since i'm missing a better pattern for attaching
context to reqwest middleware errors in
https://github.com/TrueLayer/reqwest-middleware/pull/159.
