#79 enhancement/improve enginebuilder error message (#80)

* test, and fix

* undo unused change

* neaten up hint message

* not using list of candidates in message any more

* shorten test name

src/Memstate.Core/Configuration/Providers.cs

@@ -56,7 +56,9 @@ protected T AutoResolve()
                     return provider;
                 }
             }
-            throw new Exception("Autoresolve failed for " + typeof(T));
+
+            var hint = "Please check to see if you need to add a reference to 'Memstate.Wire', or 'Memstate.JsonNet'. Adding any of these two nuget packages will automatically use either package for serialisation.";
+            throw new Exception($"Autoresolve failed for {typeof(T)}. {hint}");
         }
 
         internal class Registry : Dictionary<string, Func<T>>

src/Memstate.Test/EngineBuilderTests/BuildTests.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using Memstate.Configuration;
+using NUnit.Framework;
+using System.Threading.Tasks;
+
+namespace Memstate.Test.EngineBuilderTests
+{
+    [TestFixture]
+    class BuildTests
+    {
+        internal class TestSerializers : Serializers
+        {
+            internal TestSerializers()
+            {
+                RegisteredProviders.Remove("Wire");
+                RegisteredProviders.Remove("NewtonSoft.Json");
+            }
+        }
+
+        [Test]
+        public async Task WhenNoSerializers_BuildShould_ReturnErrorMessageContainingSuggestedPackagesToReference()
+        {
+            // pretend this is a new users project and no defaults could be loaded
+            // remove the default storage providers that will be picked up 
+            // because this test project  references Wire and JsonNet,
+            var config = Config.Reset();
+            config.Serializers = new TestSerializers();
+
+            try
+            {
+                var engine = await new EngineBuilder().Build<List<int>>();
+                Assert.Fail("Should not get here");
+            }
+            catch (Exception ex)
+            {
+                StringAssert.Contains("Please check to see if you need to add a reference to 'Memstate.Wire', or 'Memstate.JsonNet'. Adding any of these two nuget packages will automatically use either package for serialisation.", ex.Message);
+            }
+        }
+    }
+}