feat: merge nested observer errors instead of nesting

[logaretm]

This PR introduces a different strategy to handle nested observers which have been very confusing. The observers' errors were also nested, but this PR merges errors that make the structure predictable. Which allows us to tackle #1923 and #2484

closes #2528

[codecov]

Codecov Report

Merging #2530 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2530      +/-   ##
==========================================
+ Coverage   95.72%   95.72%   +<.01%     
==========================================
  Files          50       50              
  Lines        1122     1124       +2     
  Branches      162      162              
==========================================
+ Hits         1074     1076       +2     
  Misses         48       48

Impacted Files	Coverage Δ
src/components/Observer.ts	92.77% <100%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9c85b99...1fac456. Read the comment docs.

[schmulschubiak]

But nesting had a big advantage in the past. I could get very fast via errors.errors[vid].length, if a nested observer has errors, for showing error-icons outside of the observer-dom-element (like in tab panes). Now vid has no need. How can I determinate if a nested observer has errors?