Backend/Marshalling: don't return null in ExtractType

Second part of the fix for bug 242.

src/GWallet.Backend/Marshalling.fs

@@ -149,11 +149,15 @@ module Marshalling =
         let wrapper = JsonConvert.DeserializeObject<MarshallingWrapper<obj>> json
         if Object.ReferenceEquals(wrapper, null) then
             failwith <| SPrintF1 "Failed to extract type from JSON (null check): %s" json
-        try
-            Type.GetType wrapper.TypeName
-        with
-        | :? NullReferenceException as _nre ->
-            failwith <| SPrintF1 "Failed to extract type from JSON (NRE): %s" json
+        let res =
+            try
+                Type.GetType wrapper.TypeName
+            with
+            | :? NullReferenceException as _nre ->
+                failwith <| SPrintF2 "Failed to extract type '%s' from JSON (NRE): %s" wrapper.TypeName json
+        if isNull res then
+            failwith <| SPrintF2 "Failed to extract type '%s' from JSON (result was null): %s" wrapper.TypeName json
+        res
 
     // FIXME: should we rather use JContainer.Parse? it seems JObject.Parse wouldn't detect error in this: {A:{"B": 1}}
     //        (for more info see replies of https://stackoverflow.com/questions/6903477/need-a-string-json-validator )