Asp.net - Enforcing Database Constraints In Application Code

I'm working on my first real asp.net web application and I'm kind of stumped on the best place and method to trap and handle database constraint violations. Suppose I have a unique constraint on a column and a user inputs something that would violate that unique constraint. Do I catch it in the business layer by making a call to the database to check of the value exists for that column or do I let it go all the way to the database and let it throw an exception and handle that in my application?

I don't like the latter answer- because it makes sense to handle as much as you can in the business layer before it reaches the database. But it's also somewhat problematic as my application is driven by procedures that implement basic CRUD operations on all tables. So in order for me to enforce uniqueness in the business layer, I need to create a procedure for every possible constraint so I can do a look up before an update or insert. That just seems tedious... and I'm duplicating business rules as they exist in both the database and the business layer, so if something changes, not only do I have to change the database, but I have to change the procedure and the application code.

So is there a right way or a good way to enforce these database constraints in application code or should I be looking at a way to catch exceptions thrown by the database and present them in a user-friendly way?