#280 Add headers to response (#286)

* #280 can now add response headers inc trace id, now need to consolidate the header place holder stuff

* #280 changed port for linux tests

* #280 lots of hacking around to handle errors and consolidate placeholders into one class

src/Ocelot/Infrastructure/CouldNotFindPlaceholderError.cs

@@ -0,0 +1,12 @@
+using Ocelot.Errors;
+
+namespace Ocelot.Infrastructure
+{
+    public class CouldNotFindPlaceholderError : Error
+    {
+        public CouldNotFindPlaceholderError(string placeholder) 
+            : base($"Unable to find placeholder called {placeholder}", OcelotErrorCode.CouldNotFindPlaceholderError)
+            {
+            }
+    }
+}