Fix missing error messages on variable fields

[D8H]

Changes

• Errors are displayed for any undeclared child variables like they already are in the event sheet.
• Error inside brackets are now displayed (but it still doesn't display the error location).

Regressions

• This error is no longer displayed but I think it's ok because the editor doesn't allow to declare variables with space in their name.

The variable name contains a space - this is not recommended. Prefer to use underscores or uppercase letters to separate words.

• The error messages no longer have a link to open the variable editor but I think it's ok because the auto-completion covers the error message anyway.