Move error handling to Connection

The adapter shouldn't be responsible for translating errors
into the proper exception.

Octokit.Tests/Http/ConnectionTests.cs

@@ -1,10 +1,13 @@
 using System;
 using System.Linq;
+using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
 using NSubstitute;
 using Octokit.Http;
+using Octokit.Tests.Helpers;
 using Xunit;
+using Xunit.Extensions;
 
 namespace Octokit.Tests.Http
 {
@@ -116,6 +119,26 @@ namespace Octokit.Tests.Http
                 Assert.NotNull(resp.ApiInfo);
                 Assert.Equal("user", resp.ApiInfo.AcceptedOauthScopes.First());
             }
+
+            [Theory]
+            [InlineData(HttpStatusCode.Forbidden)]
+            [InlineData(HttpStatusCode.Unauthorized)]
+            public async Task ThrowsAuthenticationExceptionExceptionForAppropriateStatusCodes(HttpStatusCode statusCode)
+            {
+                var httpClient = Substitute.For<IHttpClient>();
+                IResponse<string> response = new ApiResponse<string> { StatusCode = statusCode};
+                httpClient.Send<string>(Args.Request).Returns(Task.FromResult(response));
+                var connection = new Connection(ExampleUri,
+                    Substitute.For<ICredentialStore>(),
+                    httpClient,
+                    Substitute.For<IJsonSerializer>());
+
+                var exception = await AssertEx.Throws<AuthenticationException>(
+                    async () => await connection.GetAsync<string>(new Uri("/endpoint", UriKind.Relative)));
+
+                Assert.Equal("You must be authenticated to call this method. Either supply a login/password or an " +
+                             "oauth token.", exception.Message);
+            }
         }
 
         public class TheGetHtmlMethod