Print out the correct missing step name

[bmxpert1]

Previously this code:

step "I am logged in" do
  step "this step doesn't exist"
end

would have output:

No such step: 'I am logged in'

This patch uses the value set to Turnip::Pending instead of the parents step for the output, making the output:

No such step: 'This step doesn't exist'

[jnicklas]

Nice, thanks!