In Entity Framework and the repository pattern, where should DB exceptions be handled?

how would you design this scenario (using Entity Framework 4.1, Code First and repository pattern): the Visual Studio solution contains the following projects

Solution
|-Web Application Project
|-DAL Project
|-Model Project

So in the Model Project there are various classes. Suppose we have in there a class called User with the following definition (stripped down):

public class User{

    [Key]
    public int UserId { get; set; }

    ....

    //this property has a unique constraint created in a custom DB Initializer class
    public string email { get; set; }

    ....
}

In the DAL Project reside the repository methods (Insert, Update etc.) and also the Initializer class:

public class MyDatabaseInitializer : IDatabaseInitializer<MyDatabase>
{
    public void InitializeDatabase(MyDatabase context)
    {
        try
        {
            if (!context.Database.Exists())
            {
                context.Database.Create();
                context.Database.ExecuteSqlCommand(
                    "ALTER TABLE Users ADD CONSTRAINT uc_Email UNIQUE(Email)");
            }
        }
        catch (Exception ex)
        {
            throw ex.InnerException;
        }
    }
}

The Commit method of my Unit of Work class looks like this:

public string Commit()
{
    string errorMessage = string.Empty;

    try
    {
        Database.Commit();
    }
    catch (DbUpdateException updExc)
    {                                 
        errorMessage = updExc.InnerException.Message;            
    }                       

    return errorMessage;
}

As you see I'm handling DbUpdateException in the Commit() method of the Unit of Work class; this means for each class which could cause an update error, this would be handled here.

Suppose one inserts the User records with the following Data:

(UserId,....,Email,...)
1, ... , person1@mail.com , ...
2, ... , person1@mail.com , ...

It 's obvious that this will cause a DbUpdateException to occur. Of course this can be caught and propagated to the place where it should show up. I have the feeling that this design is completely wrong:

1. Validation should occur for each property separately: shouldn't this be true also for the uniqueness of values of field? Does this mean that I have to merge DAL and MODEL into one project?

2. How would I handle errors caused by a violation of the uniqueness for fieldA in table A, fieldB in table B, fieldC in table C? Using a generic error message "The value already exists" or "Uniqueness violation" is not very descriptive!

3. Should I insert another project-Business layer which takes care of such error handling?

4. Should I handle the errors in the (ASP.NET MVC) Action/Controller which does the update?

5. How to handle a proper error message in a multi language application?