How to tell Web API 2.1 to allow errors to propagate up the MessageHandler pipeline so all exceptions can be converted to HttpResponseMessages in one place.

gistfile1.cs

    class Program
    {
        static void Main(string[] args)
        {

            var server = WebApp.Start("http://localhost:1002/", (app) =>
            {
                var config = new HttpConfiguration();
                config.MapHttpAttributeRoutes();
                config.Services.Replace(typeof(IExceptionHandler), new RethrowExceptionHandler());
                config.MessageHandlers.Add(new ErrorTrapHandler());
                app.UseWebApi(config);
            });


            Console.ReadLine();
            server.Dispose();

        }
    }
    public class RethrowExceptionHandler : IExceptionHandler {
        public Task HandleAsync(ExceptionHandlerContext context, CancellationToken cancellationToken)
        {
            return Task.FromResult(0);
        }
}

    public class ErrorTrapHandler : DelegatingHandler
    {
        protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            try
            {
                return await base.SendAsync(request, cancellationToken);
            }
            catch (Exception ex)
            {
                return new HttpResponseMessage
                {
                    ReasonPhrase = ex.Message,
                    StatusCode = HttpStatusCode.InternalServerError
                };
            }
        }
    }

here is an example where your error handling would fail:

public class MyFormatter : MediaTypeFormatter
{
    public override bool CanReadType(Type type)
    {
        return true;
    }

    public override bool CanWriteType(Type type)
    {
        return true;
    }

    public override Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content,
        TransportContext transportContext)
    {
        throw new Exception("cannot write to stream");
    }
}

        config.Formatters.Clear();
        config.Formatters.Add(new MyFormatter());

In your example the error stays unhandled and a Framework-generated 500 comes down.
If you use a proper exception handler it's handled correctly, and a custom HttpResponseMessage can be set:

public class ProperExceptionHandler : IExceptionHandler
{
    public Task HandleAsync(ExceptionHandlerContext context, CancellationToken cancellationToken)
    {
        context.Result = new ResponseMessageResult(new HttpResponseMessage
        {
            ReasonPhrase = context.Exception.Message,
            StatusCode = HttpStatusCode.InternalServerError
        });
        return Task.FromResult(0);
    }
}