Add a feature to display JSON errors

Perhaps this indicates that it would be useful to port the following from the D7 version of Feeds Extensible Parsers:

Also needs automated tests.

Ok, I rewrote the patch and updated the existing tests, which already cover this exception. I also updated the invalid JSON test by using a common JSON error (adding an extra comma at the end of a pair) instead of a plaintext string so that the test expectation is more clear.

The debug feature is interesting, but for this specific issue, I think it makes sense to simply show the bad JSON without asking the user for their preference. When the user is importing JSON, and it fails, they probably always want to know what went wrong.

There is a PHP variable used in a translatable string. See MR for details.

I fixed the exception translation and I removed StringTranslationTrait since translating exceptions was its only use.

I rebased the code to see if there were any phpcs errors and luckily there aren't.

I also wondered if instead of displaying the whole JSON, if perhaps the error message could be more specific on where the JSON is wrong, but not sure if there is an easy way to display that. json_last_error_msg() does not give any useful info on a syntax error at least.

So I merged the code. We could open a follow-up if you have ideas on how the JSON error could perhaps be more specific.

Automatically closed - issue fixed for 2 weeks with no activity.