except that 'Z' is still allowed in front of a '+/-HH:MM' timezone.  I
left that in since it was used elsewhere in the code, and was probably
harmless, whereas the other parse problems caused dates like "2003-2-5"
to be accepted as valid and silently converted into "2003-01-01".

Also re-exposed the DateTime-specific exceptions as attributes of the
DateTime class.  During their recent converted from strings to classes,
they were removed.