You cannot have it both ways. Either you have constrains or you don't.

>My UI allows user to enter the data; so there is a potential for them to enter wrong value; nothing I can about it. If I give them only combo boxes for selecting from pull-down lists some won't be happy. Thank you for your input.
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>>Make sure that your front end does not allow such situations. After that you can treat such errors as exemptions and log them as other errors.