[LIMS-2066] More descriptive error message for sample with parent/child

[gfrn]

JIRA ticket: LIMS-2066

Summary:

This adds a more descriptive error message for samples which can't be deleted because they have a parent/child sample associated

Changes:

• More descriptive error message for sample with parent/child

To test:

• Send a DELETE to /samples/1877, check if 409 is returned

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.64%. Comparing base (0aea769) to head (460eec7).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/scaup/crud/samples.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   94.68%   94.64%   -0.04%     
==========================================
  Files          42       42              
  Lines        1767     1774       +7     
==========================================
+ Hits         1673     1679       +6     
- Misses         94       95       +1     

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