More informative error messages for actions builder

The goal of this PR is to append a list of Validation errors to the error message.

* Given action `accept` which changes `Document` entity and attemts to persist the change useing the default `saveObject` method.
* Assuming the validation fails, becouse `number` field is expected to be not empty and `reciever` should be at least 3 characters long.

The following Flash error message would be created:

Before this PR:

> Could not accept Document.

After this PR:

> Could not accept Document.
>
>  number This field should not be blank.
>  reciever This field should contain at least 3 characters.

Note: *The exact error messages could be diffrent. This example is just to illustrate how the list of validation errors is appended to the original message.*