ENH: properly trap incorrect enumerations (#1372)

- the Enum::readEntry() method was previously as bit sloppy with
  respect to the enumeration that it accepted. If the input was
  non-mandatory, typos would go unnoticed. Now tighten things so that
  if an enumeration is found, it must also be valid.

STYLE: remove unused/deprecated Enum::lookupOrFailsafe() method

- this was only used in a few places internally in 1712 and 1806 but
  has since then been superseded by getOrDefault() with an optional
  'failsafe' flag.