Controller -> Service -> Repository

ViewModel to Model mapping -> Validation and other business logic -> CRUD operations

readonly IClientService _clientService;

public ClientsController(IClientService clientService)
{
    _clientService = clientService;
}

var client = new Client();
viewModel.MapTo(client);
_clientService.Insert(client);

public class MyException : Exception
{
    public MyException()
    {
    }

    public MyException(string message)
        : base(message)
    {
    }

    public MyException(string message, Exception inner)
        : base(message, inner)
    {
    }
}

public void Insert(Client client)
{
    if (client == null)
    {
        throw new MyException("This thing's not right..");
    }

try
{
    _clientService.Insert(client);
}
catch (MyException)
{
    // Add error to ModelState perhaps
}

protected override void OnException(ExceptionContext exceptionContext)
{
    exceptionContext.ExceptionHandled = true;

    if (exceptionContext.Exception is MyException)
    {
        // Perhaps add to ModelState
    }
    else
    {
        // Add a generic error message to ModelState and log error
    }

    filterContext.Result = new ViewResult
    {
        ViewName = ...
    };
}