Refactoring/renaming of validation

- validateInput renamed to validate
- validateCondition renamed to need
- Removed ability to provide "bare" conditions. It is
  still possible to fail validation silently by passing
  FALSE as the second argument to need()
- Rather than using a two-element list to convey results,
  use a single result protocol; NULL is success, FALSE is
  silent failure, string is failure with message
- Tweak "missing input" semantics, add tests