[Form] getErrorsAsArray for Form class

With this new handy method Form class is now able to provide all validation errors of a form  (and its child forms) in a beautiful associative array tree that can, for example, easily be serialized to JSON for API responses and such. It seems quite a few people (including me) have had problems with the lack of this feature. At least this is the impression I had while browsing through questions in stackoverflow etc.

Implementation is similar to existing helper method ```getErrorsAsString``` so this one should behave however it behaves.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes (...I think. There were errors/fails but not caused by my code)
| Fixed tickets |
| License       | MIT
| Doc PR        |

So... Any future for this?