Fix error checking in backend contracts

[AlekSi]

Description

defer was used incorrectly.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3031 (0cb6140) into main (f38331f) will increase coverage by 0.07%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3031      +/-   ##
==========================================
+ Coverage   74.19%   74.27%   +0.07%     
==========================================
  Files         386      386              
  Lines       21110    21120      +10     
==========================================
+ Hits        15663    15686      +23     
+ Misses       4494     4481      -13     
  Partials      953      953              

Impacted Files	Coverage Δ
internal/backends/backend.go	100.00% <100.00%> (ø)
internal/backends/collection.go	100.00% <100.00%> (ø)
internal/backends/database.go	100.00% <100.00%> (ø)
internal/backends/error.go	62.85% <100.00%> (+12.85%)	⬆️

... and 3 files with indirect coverage changes

Flag	Coverage Δ
integration	70.67% <95.74%> (+0.06%)	⬆️
mongodb	5.41% <0.00%> (-0.01%)	⬇️
pg	60.43% <0.00%> (-0.03%)	⬇️
shard-1	54.21% <95.74%> (+0.03%)	⬆️
shard-2	36.87% <95.74%> (+0.03%)	⬆️
shard-3	57.10% <95.74%> (+0.07%)	⬆️
sqlite	46.53% <95.74%> (+0.07%)	⬆️
unit	24.23% <4.25%> (-0.02%)	⬇️

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