fix: Filtering on unique index if there is no match

[islamaliev]

Relevant issue(s)

Resolves #2176

Description

Make unique index distinguish between key-not-found and other errors.

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Unit and integration tests

Specify the platform(s) on which this was tested:

• MacOS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f7a4166) 74.21% compared to head (dc819ed) 74.16%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2177      +/-   ##
===========================================
- Coverage    74.21%   74.16%   -0.06%     
===========================================
  Files          256      256              
  Lines        25366    25368       +2     
===========================================
- Hits         18825    18812      -13     
- Misses        5263     5275      +12     
- Partials      1278     1281       +3     

Flag	Coverage Δ
all-tests	74.16% <100.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
db/fetcher/indexer_iterators.go	82.84% <100.00%> (+0.78%)	⬆️

... and 11 files with indirect coverage changes

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[jsimnz]

LGTM