Raise error instead of warning when writing image with non-orthogonal direction matrix to nifti.

### Description

When saving an sitk.Image to NIFTI, the direction matrix get coerced to an orthogonal matrix. A warning is raised, but in my opinion this behavior is rather unexpected. This can lead to subtle and hard-to-trace bugs if the direction matrix was not orthogonal. Therefore, I suggest to raise an error in this case. This backwards-incompatible change could be introduced in `itk 6.0`.

### Expected behavior

An error should be raised, e.g.:
```
Direction matrix is not orthogonal. Writing to NIfTI not supported.
```

If the user want to save the image as NIfTI, the user should also explicitly modify the direction matrix such that [itk can work with it](https://discourse.itk.org/t/saving-non-orthogonal-volume-in-nifti-format/2760/22).

### Actual behavior

Currently, only a warning is being raised, see https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/IO/NIFTI/src/itkNiftiImageIO.cxx#L2335.

```
Non-orthogonal direction matrix coerced to orthogonal.
```

### Additional Information

- see https://github.com/SimpleITK/SimpleITK/issues/2360 for a code example in SimpleITK
- itk version 5.4.4.