Make parse_datetime raise ISO8601Error if no 'T'

- Make parse_datetime raise ISO8601Error if time designator 'T' is missing
  as before it would raise a ValueError that was hard to interpret.
- Alter test_parse for test_datetime to check that bad inputs raise an
  ISO8601 error. The other tests did this, but test_datetime didn't.
- Add test case where the 'T' time designator is missing from input
  datetime strings.