Forms: allow previewing and modifying validation errors

> 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

Start by locating the form block settings and the existing error-state rendering described in the issue. Define how error-state previewing and separate typography and color controls should work, then verify that the error text can be previewed and styled without custom CSS.

Written by the indexing model from the issue text.