Refactor item errors

[vbanos]

Previously, when we had an item error we returned fmt.Errorf("item is fresh but has children"). and then in the unit tests we had to use
expected: errors.New("item is fresh but has children"

There is no need to define an error multiple times. We define:

ErrItemFreshHasChildren = errors.New("item is fresh but has children")

and then use ErrItemFreshHasChildren everywhere.

This way, we can easily change the error text in one place and avoid breaking the unit tests.

Also, we could avoid doing error string comparison and use errors.Is but we can't do it right now because the tests have some special error cases and its not straightforward.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.33%. Comparing base (0837f4d) to head (351fa5b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/models/item.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
- Coverage   56.42%   56.33%   -0.09%     
==========================================
  Files         130      130              
  Lines        8091     8091              
==========================================
- Hits         4565     4558       -7     
- Misses       3161     3167       +6     
- Partials      365      366       +1     

Flag	Coverage Δ
e2etests	40.61% <0.00%> (-0.04%)	⬇️
unittests	29.33% <87.50%> (-0.06%)	⬇️

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[NGTmeaty]

Thank you!