added useful reminder about form.vars.errors into UPGRADE-2.5 notes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

PR is just about upgrading UPGRADE-2.5 notes.
We've been upgrading from 2.4 to 2.5.3 and missing this part (upgrading templates)