Errors ergonomics improvement

[japanese-goblinn]

Description

• change let to var in errors
  • I saw that overall project uses var instead of let but in my opinion at least errors should be fully immutable because I do not see any scenario where you should mutate and error (so as there were no error mutations in the project).
• Change String(data:encoding:) to String(decoding:as:). It's better to provide info with � symbols if not UTF8 rather than an empty string.
• use RawValue in errors instead of actual RawValue type (e.g. instead on Int use Struct.RawValue). Easier to refactor code

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation comments
• [x] My changes generate no new warnings

[japanese-goblinn]

@stackotter please, check

[japanese-goblinn]

@stackotter thanks for this kind of words, really appreciate it