Feature/inject error mapper (#562)

* added delegate to select last handler

* #529 implemented a way we can inject the last delegating handler

* wip - moving code

* #529 removed loads of qos code and moved it into Ocelot.Provider.Polly

* #529 can now inject http client expcetions to ocelot errors mappers and updated docs

test/Ocelot.UnitTests/Requester/HttpExeptionToErrorMapperTests.cs

@@ -0,0 +1,52 @@
+namespace Ocelot.UnitTests.Requester
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Threading.Tasks;
+    using Microsoft.Extensions.DependencyInjection;
+    using Ocelot.Errors;
+    using Ocelot.Requester;
+    using Responder;
+    using Shouldly;
+    using Xunit;
+
+    public class HttpExeptionToErrorMapperTests
+    {
+        private HttpExeptionToErrorMapper _mapper;
+        private readonly ServiceCollection _services;
+
+        public HttpExeptionToErrorMapperTests()
+        {
+            _services = new ServiceCollection();
+            var provider = _services.BuildServiceProvider();
+            _mapper = new HttpExeptionToErrorMapper(provider);
+        }
+
+        [Fact]
+        public void should_return_default_error_because_mappers_are_null()
+        {
+            var error = _mapper.Map(new Exception());
+
+            error.ShouldBeOfType<UnableToCompleteRequestError>();
+        }
+
+        [Fact]
+        public void should_return_error_from_mapper()
+        {
+            var errorMapping = new Dictionary<Type, Func<Exception, Error>>
+            {
+                {typeof(TaskCanceledException), e => new AnyError()},
+            };
+
+            _services.AddSingleton(errorMapping);
+
+            var provider = _services.BuildServiceProvider();
+
+            _mapper = new HttpExeptionToErrorMapper(provider);
+
+            var error = _mapper.Map(new TaskCanceledException());
+
+            error.ShouldBeOfType<AnyError>();
+        }
+    }
+}