How do I catch a SqlException thrown by RAISERROR called in a trigger set off by Entity Framework 4?

I have Entity Framework set up to update a table. The update is intercepted by an instead-of trigger, which calls RAISERROR:

CREATE TRIGGER mySchema.UpdateBusinessObjects
ON mySchema.BusinessObjects
INSTEAD OF UPDATE
AS
    RAISERROR(''test error from SQL'',16,1)
    RETURN

In my repository class, I'm attempting to catch the SqlException generated by the RAISERROR in SQL:

public void SaveBusinessObject(BusinessObject b) {
    try {
         repo.Entry(b).State = EntityState.Modified;
         repo.SaveChanges();
    } catch (SqlException ex) {
         // handle exception here
    }
}

The problem is that C# isn't catching the SqlException; it gets passed up to the caller as an unhandled exception ("SqlException was unhandled by user: test error from SQL"). What?!

It looks like EF's SaveChanges() somehow is passing the exception up over my try catch block. I have tried switching my catch statement to catch (Exception ex) in case the EF exception is somehow more general, but I still get an unhandled SqlException. Am I missing something simple here? What's the problem with the SaveChanges() method?