[Form] Improved exception message if the data class is not found

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12359
| License       | MIT
| Doc PR        | -
And maybe `Class "%s" not found.` reads better.