Error handling done at form submission level, validation check

try using javascript:

http://stackoverflow.com/questions/13339827/how-to-add-javascript-validation-to-choice-field-and-check-for-a-valid-option

http://social.msdn.microsoft.com/Forums/sharepoint/en-US/6a1d5e71-19b5-4dba-a792-c6bb04e872d3/sharepoint-list-using-javascript-to-validate-custom-list-fields

just check if the field in NULL then do some necessary error messages

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----------------------- Sharepoint Newbie