Refactor errors

[zemanlx]

I was looking at the codebase and thinking if I can make some part a bit nicer. Here is my first attempt.

I have:

• added unit tests for ErrorFilter function to lock behaviour and be sure later that I am not changing it.
• removed dead code
• refactored ErrorFilter function to
  • return early
  • avoid else as much as possible
  • not duplicate work
  • decrease number of indented code
  • decrease brain buffer when reading code flow

I can do more PRs like this one if you find it useful.