Catch and print exceptions

Because of the way this script is invoked from str_cmds.js (using js.exec)
any exceptions were just silent failures, making debugging issues
(e.g. the recent regression with the global ascii() function) difficult.

exec/mqtt_spy.js

@@ -87,6 +87,7 @@ if(!mqtt.subscribe(terminal_topic)) {
 }
 print("*** Synchronet MQTT Spy on Node " + node_num + ": Ctrl-C to Abort ***\r\n");
 
+try {
 	while(!js.terminated) {
 		var msg = mqtt.read(/* timeout: */10, /* verbose: */true);
 		if(msg) {
@@ -127,5 +128,8 @@ while(!js.terminated) {
 			}
 		}
 	}
+} catch(e) {
+	print(e);
+}
 print();
 print("Done spying");