AR or Statesman, error handling patterns in controller (sort of pseudocode)

ar-or-statesman-error.rb

# ActiveRecordish pattern
def create
  @thing = Thing.create!(thing_params)
  redirect_to @thing
rescue Thing::Invalid #???
  # flash error
  # re-render
end

def create
  # if (@thing = Thing.create(thing_params))
  #   redirect_to @thing
  # else
  #   flash[:error] = 'bad thing'
  #   # render show page again
  # end
end

# similar for statesman I think?

def cancel
  # unless @evaluation.transition_to(:cancelled)
  #   add flash error
  # end

  begin
    @evaluation.transition_to!(:cancelled)
  rescue Statesman::TransitionFailed, Statesman::GuardFailedError
    # add flash error
  end

  redirect_to @project
end