FIO-9668: Fix custom error messages are not highlighted

[mikekotikov]

Link to Jira Ticket

https://formio.atlassian.net/browse/FIO-9668

Description

What changed?

I've added "level" field to be assigned to custom errors, since it's needed in formio.js, as it filters out all non-"error" level messages.

Why have you chosen this solution?

Our other errors (required, min, max, etc.) already have that "level" assigned and work fine.

Breaking Changes / Backwards Compatibility

Use this section to describe any potentially breaking changes this PR introduces or any effects this PR might have on backwards compatibility

Dependencies

Use this section to list any dependent changes/PRs in other Form.io modules

How has this PR been tested?

I've added an automated test case.

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above