Handle submission actor's result

Description

Previously, Episerver Forms did not handle submission actor’s result (actors actually returned results but they were not used).
This feature allows actors to:

• Return signal to cancel form submission in case actor running fails
• Return error message which can be displayed to visitors
  There are some changes when implementing actors in order for the above to work:
  1. Actors must implement ISyncOrderedSubmissionActor. Actors implementing this interface will run synchronously in ascending order, regardless of IsSyncedWithSubmissionProcess value (we force the actor to run synchronously because we cannot control the result of async actors).
  2. Actors must return object instance of a class which implements EPiServer.Forms.Core.PostSubmissionActor.Internal.ISubmissionActorResult.
  By implementing this interface, the returned result will have two properties:
• CancelSubmit (boolean): determine whether the form submission should be canceled or not.
• ErrorMessage (string): this error message will be displayed to visitors.