Forms: allow previewing and modifying validation errors

[simison]

I used the block settings to change the color of the text to a different color, but you can’t change the error text color without css.

It'll probably make sense to have some mechanism to allow previewing fields in error state, and then have typography+color controls for them separately.

Internal ref p1737033540877189-slack-C0197R4UB4J

[github-actions]

This issue could use some more labels, to help prioritize and categorize our work. Could you please add at least a [Type], a [Feature], and a [Pri] label?