You can use the fluent api to specify the actions the error message suggests.
In your Context:
protected override void OnModelCreating( DbModelBuilder modelBuilder )
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<aspnet_UsersInRoles>().HasMany(i => i.Users).WithRequired().WillCascadeOnDelete(false);
}
Note that you have not included the definition for the table aspnet_UsersInRoles so this code may not work.
Another option is to remove all CASCADE DELETES by adding this
modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();
If you need more info about configuring relationships with the fluent api I suggest http://msdn.microsoft.com/en-US/data/jj591620