Added deprecation note about 'virtual' form option. Issue #12603.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12603 
| License       | MIT
| Doc PR        | 

Uncommented the trigger_error method calls for 'virtual' form option.