adjust example exception behavior to match latest spec

examples/subprotocol_server/subprotocol_server.cpp

@@ -42,11 +42,7 @@ int main() {
         s.start_accept();
 
         s.run();
-    } catch (const std::exception & e) {
+    } catch (websocketpp::exception const & e) {
         std::cout << e.what() << std::endl;
-    } catch (websocketpp::lib::error_code e) {
-        std::cout << e.message() << std::endl;
-    } catch (...) {
-        std::cout << "other exception" << std::endl;
     }
 }