DICOM redactor improvement: Adding exceptions for when DICOM file does not have pixel data

[niwilso]

Change Description

DICOM files do not necessarily always have pixel data (e.g., DICOM directory files). As a result, we need to ensure the redactor and verification engine are able to properly raise an error when such files are passed in. This PR introduces exception handling and updated tests for this.

Issue reference

This PR fixes issue #XX

Checklist

• I have reviewed the contribution guidelines
• I have signed the CLA (if required)
• My code includes unit tests
• All unit tests and lint checks pass locally
• My PR contains documentation updates / additions if required