Fixes#20781
Previously, when `GetProvisionerKey()` failed, the actual error was
swallowed:
```
error: unable to get provisioner key details
```
Now the actual error is preserved using error wrapping, so users can see
the real cause (e.g., 404 Not Found, connection refused, invalid key,
etc.):
```
error: unable to get provisioner key details: GET https://...: 404 Not Found
```
This makes it much easier to diagnose configuration issues.
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