Reworked the interaction for entry of scalar, and general calculation of the value
- Show validation error message for proper feedback
- Trap ^C properly, as done for the list-case already. Scalar was forgotten.
- Report the ^C as 'undefined value'
- Remember the undefined-ness and have additional attempts to retrieve the value abort early
  This prevents retries of computing the value where we cannot succeed.
  It also prevents additional attempts at interaction.
Bumped version to 0.10.