BE: KC: Expose Kafka Connect validation errors in the UI

[yeikel]

What changes did you make? (Give an overview)

Closes #681

Example:

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

A picture of a cute animal (not mandatory but encouraged)

[Haarolean]

Thanks much for implementing this, experiencing this issue wasn't very pleasant when working with KC :)

[yeikel]

@Haarolean Please take a second look 🙇

I opted for the first suggestion and used @SuppressWarnings("checkstyle:LineLength") because I think that checkstyle should not limit the language features we get access to or decrease the readability of the code. Text blocks, in my opinion, make the test more readable than workarounds using concatenation

[yeikel]

@Haarolean Are the current test failures a known problem?

[Haarolean]

@yeikel yeah it's a flaky one

[Haarolean]

@yeikel thank you!