Bug: Stepform swallows validation errors

[Ruben van Leeuwen]

Bug
When a workflow reaches a suspended state where it asks for user input it displays a form for the user to supply said input. This is known in code as a stepform. The stepform currently swallows the validation errors it might generate. Supplying invalid data sets the form in 'loading' state and doesn't displays errors or gets out if this state

Acceptance
Make the stepform display validation errors - like it does at the start of a workflow - and give the user the ability to correct errors and resubmit.