Use toSpanned in validation error dialog text

[FikriMilano]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2797

Description
Use toSpanned in validation error dialog text

Alternative(s) considered
Another way is to remove the HTML, to only extract the text content.
But, just using toSpanned is much simpler.

Type
Bug fix

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[MJ1998]

Can you run ./gradlew spotlessapply, @FikriMilano ?

[FikriMilano]

@MJ1998 spotless applied

[MJ1998]

LGTM