tests to handle some error cases and docs

src/Ocelot/Errors/OcelotErrorCode.cs

@@ -41,5 +41,6 @@
         QuotaExceededError = 36,
         RequestCanceled = 37,
         ConnectionToDownstreamServiceError = 38,
+        CouldNotFindLoadBalancerCreator = 39,
     }
 }

src/Ocelot/LoadBalancer/LoadBalancers/CouldNotFindLoadBalancerCreator.cs

@@ -0,0 +1,12 @@
+namespace Ocelot.LoadBalancer.LoadBalancers
+{
+    using Errors;
+
+    public class CouldNotFindLoadBalancerCreator : Error
+    {
+        public CouldNotFindLoadBalancerCreator(string message) 
+            : base(message, OcelotErrorCode.CouldNotFindLoadBalancerCreator)
+        {
+        }
+    }
+}

src/Ocelot/LoadBalancer/LoadBalancers/LoadBalancerFactory.cs

@@ -28,7 +28,13 @@
 
             var serviceProvider = serviceProviderFactoryResponse.Data;
             var requestedType = reRoute.LoadBalancerOptions?.Type ?? nameof(NoLoadBalancer);
-            var applicableCreator = _loadBalancerCreators.Single(c => c.Type == requestedType);
+            var applicableCreator = _loadBalancerCreators.SingleOrDefault(c => c.Type == requestedType);
+
+            if (applicableCreator == null)
+            {
+                return new ErrorResponse<ILoadBalancer>(new CouldNotFindLoadBalancerCreator($"Could not find load balancer creator for Type: {requestedType}, please check your config specified the correct load balancer and that you have registered a class with the same name."));
+            }
+
             var createdLoadBalancer = applicableCreator.Create(reRoute, serviceProvider);
             return new OkResponse<ILoadBalancer>(createdLoadBalancer);
         }

src/Ocelot/Responder/ErrorsToHttpStatusCodeMapper.cs

@@ -45,7 +45,8 @@ namespace Ocelot.Responder
                 return 502;
             }
 
-            if (errors.Any(e => e.Code == OcelotErrorCode.UnableToCompleteRequestError))
+            if (errors.Any(e => e.Code == OcelotErrorCode.UnableToCompleteRequestError
+                || e.Code == OcelotErrorCode.CouldNotFindLoadBalancerCreator))
             {
                 return 500;
             }