Clarify error message

include/sardine.py

         method_n = "render_" + self.data.fmt
 
         def ex(__):
-            raise SardinePluginError("method not defined: " + method_n)
+            raise SardinePluginError("plugin method not defined: " + method_n)
 
         render_fn = getattr(self, method_n, ex)
         return render_fn()