Validate unused error types

[raviqqe]

Close #791.

[codecov-commenter]

Codecov Report

Merging #799 (fd10dde) into main (ae67b6f) will increase coverage by 0.08%.
The diff coverage is 97.63%.

@@            Coverage Diff             @@
##             main     #799      +/-   ##
==========================================
+ Coverage   85.32%   85.41%   +0.08%     
==========================================
  Files         316      317       +1     
  Lines       29297    29466     +169     
==========================================
+ Hits        24999    25168     +169     
  Misses       4298     4298              

Impacted Files	Coverage Δ
lib/hir-mir/src/error.rs	0.77% <0.00%> (-0.02%)	⬇️
...b/hir-mir/src/validation/unused_error_validator.rs	98.79% <98.79%> (ø)
lib/hir-mir/src/lib.rs	88.23% <100.00%> (+0.04%)	⬆️
lib/hir/src/analysis/ir/expression_visitor.rs	72.82% <0.00%> (+4.34%)	⬆️

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