Split the diff_errors logic from DicomValidator.validate_anonymised into a separate function

[p-j-smith]

Description

Part of #665
Closes PIX-78

• not necessary, but I thought it would make sense to create a new function get_new_errors that contains the logic for comparing validation errors from the original and anonymised datasets
• update tests to use the get_new_errors function

Type of change

Please delete options accordingly to the description.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Suggested Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have commented my code, particularly in hard-to-understand areas.
• I have passed on my local host device. (see further details at the CONTRIBUTING document)
• Make sure your branch is up-to-date with main branch. See CONTRIBUTING for a general example to syncronise your branch with the main branch.
• I have requested review to this PR.
• I have addressed and marked as resolved all the review comments in my PR.
• Finally, I have selected squash and merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.41%. Comparing base (d7dec18) to head (da38785).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           paul/665-dicom-validator-0.9     #676      +/-   ##
================================================================
+ Coverage                         88.40%   88.41%   +0.01%     
================================================================
  Files                                81       81              
  Lines                              3966     3972       +6     
================================================================
+ Hits                               3506     3512       +6     
  Misses                              460      460              

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[linear-code]

PIX-78