[Form] Changed Form::getErrors() to return an iterator and added two optional parameters $deep and $flatten

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #7205
| License       | MIT
| Doc PR        | -

See the changes in the UPGRADE files for more information.