print warning if the plugin cannot be loaded as it is lacking an interface

src/datasource_cache.cpp

@@ -226,6 +226,10 @@ void datasource_cache::register_datasources(const std::string& str)
 #endif 
                             registered_=true;
                         }
+                        else
+                        {
+                            std::clog << "Problem loading plugin library '" << itr->path().string() << "' (plugin is lacking compatible interface" << std::endl;                    
+                        }
                     }
                     else
                     {