Update TextLoader.cs (dotnet#129)

Make a 'not supported field type' exception more readable, so the developer could figure out why he can't load the data
This closes dotnet#128

src/Microsoft.ML/TextLoader.cs

@@ -91,7 +91,7 @@ private string TypeToName(Type type)
             else if (type == typeof(bool))
                 return "BL";
             else
-                throw new Exception("Type not implemented or supported."); //Add more types.
+                throw new System.NotSupportedException("Type ${type.FullName} is not implemented or supported."); //Add more types.
         }
 
         public ILearningPipelineStep ApplyStep(ILearningPipelineStep previousStep, Experiment experiment)