DateType.convert method needs better error handling

DateType.convert method assume you will pass in either a Date or Calendar otherwise it assumes you are passing a String that can be converted to an Integer.

If you pass in an invalid date string it will just return null and hide the error. We should either support a date string or throw an error.

There is a similar problem for other type converters, for instance IntegerType.convert returns null on invalid input but it should throw.