Treat errors caused by core/shortcode blocks like non-block issues

[delawski]

Summary

Fixes #6667

This PR no longer treats errors generated by the core/shortcode block type as if they were WordPress core issues. Instead, all types of error sources are being examined in getErrorSourceTitle function in order to find the root cause of the issue.

Before	After

On top of that, Gutenberg is returned as a Source only if it is the sole source of an error (the same logic is used in the other areas of the plugin).

Moreover, this PR fixes an issue where an inexistent source property of the blockSources object is being accessed instead of the type property, e.g.:

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 8b0e968 are ready 🛎️!

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[codecov]

Codecov Report

Merging #6675 (180066e) into develop (0478c24) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 180066e differs from pull request most recent head 79a4ed3. Consider uploading reports for the commit 79a4ed3 to get more accurate results

@@              Coverage Diff              @@
##             develop    #6675      +/-   ##
=============================================
+ Coverage      76.38%   76.45%   +0.06%     
- Complexity      6475     6476       +1     
=============================================
  Files            261      261              
  Lines          20697    20703       +6     
=============================================
+ Hits           15809    15828      +19     
+ Misses          4888     4875      -13     

Flag	Coverage Δ
javascript	57.77% <100.00%> (+0.17%)	⬆️
php	77.58% <ø> (+0.06%)	⬆️
unit	77.58% <ø> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...block-validation/components/error/error-content.js	93.33% <100.00%> (-0.22%)	⬇️
...idation/components/error/get-error-source-title.js	94.73% <100.00%> (+0.98%)	⬆️
includes/class-amp-service-worker.php	78.74% <0.00%> (ø)
includes/sanitizers/class-amp-link-sanitizer.php	85.71% <0.00%> (+13.54%)	⬆️

[westonruter]

Confirmed fix to attribution:

Before	After

[westonruter]

Looks great other than the one small issue regarding the gutenberg slug.