Improve form loading error handling

Thank you for your contribution with code! This is a good idea. I've made a review in the MR with requested improvements to the code.

Thanks for all the revised code. I'll review as soon as I can. I haven't moved this project to GitLab CI testing yet, so in the meantime it would be good to investigate why there are test failures showing for Drupal CI?

Tests passing, but 2 style issues. We still need to resolve the added module dependencies

FILE: ...lds/project/formassembly/web/modules/custom/formassembly/src/ApiMarkup.php
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FOUND 1 ERROR AND 1 WARNING AFFECTING 2 LINES
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 166 | WARNING | [x] A comma should follow the last multiline array item.
     |         |     Found: ) (Drupal.Arrays.Array.CommaLastItem)
 173 | ERROR   | [x] Concat operator must be surrounded by a single space
     |         |     (Squiz.Strings.ConcatenationSpacing.PaddingFound)