feat: allow checking multiple errors codes in `IsError`

[jooola]

Useful when one need to check multiple error codes at once, without having to cast every time.

-hcloud.IsError(err, hcloud.ErrorCodeConflict) || hcloud.IsError(err, hcloud.ErrorCodeLocked)
+hcloud.IsError(err, hcloud.ErrorCodeConflict, hcloud.ErrorCodeLocked)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.53%. Comparing base (cf9fdaf) to head (418e4cf).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #491 +/- ## ======================================= Coverage 71.53% 71.53% ======================================= Files 46 46 Lines 3910 3910 ======================================= Hits 2797 2797 Misses 699 699 Partials 414 414 ```

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