🐛 webhook: clarify CertDir default and wrap serving-cert load error

[alliasgher]

Summary

Addresses the two small, agreed-upon asks from #900:

1. Better docs. The `CertDir` default — `/k8s-webhook-server/serving-certs` — depends on `os.TempDir()`, which is `/tmp` on most Linux installs but is `TMPDIR`-dependent elsewhere. Maintainer asked to keep the default stable but update the comment to explain this and to nudge operators toward setting CertDir explicitly (usually to a mounted Secret path). Done.
2. Helpful error. When `certwatcher.New` fails because the certs weren't where `CertDir` was pointed, users got a bare `open ...: no such file or directory`. Wrap the error so it names the paths that were attempted, prints the effective `CertDir`/`CertName`/`KeyName`, and points at `webhook.Server.Options.CertDir` as the thing to fix. The underlying error is preserved via `%w`.

Before

```
open /tmp/k8s-webhook-server/serving-certs/tls.crt: no such file or directory
```

After

```
failed to load serving cert from "/tmp/k8s-webhook-server/serving-certs/tls.crt" / "/tmp/k8s-webhook-server/serving-certs/tls.key" — did you mount the certificate files at webhook.Server.Options.CertDir? (CertDir="/tmp/k8s-webhook-server/serving-certs", CertName="tls.crt", KeyName="tls.key"): open /tmp/k8s-webhook-server/serving-certs/tls.crt: no such file or directory
```

Fixes #900

Tests

`go build ./pkg/webhook/...` and `go vet ./pkg/webhook/...` pass. No behavior change except the added context on the error path.

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[k8s-ci-robot]

Hi @alliasgher. Thanks for your PR.

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[troy0820]

/ok-to-test
/hold

[alliasgher]

/retest

[sbueringer]

/retest

[sbueringer]

Looks like this needs some test fixes, PTAL

[alliasgher]

Thanks @sbueringer. Fixed in 0e057fb — the builder webhook tests gate on os.IsNotExist(err) to tolerate the missing serving cert, but os.IsNotExist doesn't unwrap, so the new fmt.Errorf(...: %w) wrapper no longer matched. Switched those checks to errors.Is(err, fs.ErrNotExist), which unwraps and still matches the underlying os.PathError. TestBuilder passes locally with envtest.

[sbueringer]

Thanks @sbueringer. Fixed in 0e057fb — the builder webhook tests gate on os.IsNotExist(err) to tolerate the missing serving cert, but os.IsNotExist doesn't unwrap, so the new fmt.Errorf(...: %w) wrapper no longer matched. Switched those checks to errors.Is(err, fs.ErrNotExist), which unwraps and still matches the underlying os.PathError. TestBuilder passes locally with envtest.

So this PR would break folks that are using os.IsNotExist.

I'm not really sure if that change in error message is worth it. I don't think a lot of folks are using os.IsNotExist, but I also don't think that the current error reporting is a real problem that folks have.

@alvaroaleman WDYT?

[alvaroaleman]

@alvaroaleman WDYT?

Yeah I wouldn't change it