[Form] The trace of form errors is now displayed in the profiler

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5607
| License       | MIT
| Doc PR        | -

This is a follow-up PR for #12052. With this change, the full trace of form errors is now displayed in the web debugger:

![error](https://cloud.githubusercontent.com/assets/176399/4419637/85facd14-456d-11e4-8c70-0e8802a586ec.png)

If a violation was caused by a TransformationFailedException, the exception is now accessible through the `getCause()` method of the violation. Additionally, you can access `Form::getTransformationFailure()` to retrieve the exception.