Improve error messaging for JIRA integration connection tests

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Problem to solve

When saving settings for integrations such as the JIRA service, we currently test the entered settings before saving. The error messages when the test fails are not particularly helpful such as the generic "Test Failed" message reported in in #13423 (closed).

We should provide more detailed information when a connection test fails so that the user has more information to fix the problem. Ideally the solution will apply to other integrations as well.

The current error message is as follows.

gitlab.com-failed-to-validate-url

The error message for the wrong credentials case when it is allowed to enforce the change is as follows.

gitlab.com-failed-to-validate-cred

Intended users

Further details

Proposal

We should update the copy in this error to make it clearer to the user what went wrong and what their next steps should be.

[TBD BY UX]

Testing

Links / references

Edited by 🤖 GitLab Bot 🤖