Sentry shutdown message fix and telemetry config option example (#505)

* Only print sentry shutdown message if TelemetryService exists

Fix for #504

* Config example for telemetry option

Author: fignew

etc/et.cfg

@@ -9,3 +9,4 @@ port = 2022
 verbose = 0
 silent = 0
 logsize = 20971520
+telemetry = 1

src/terminal/TelemetryService.cpp

@@ -76,8 +76,8 @@ class TelemetryDispatcher : public el::LogDispatchCallback {
 };
 
 void shutdownTelemetry() {
-  cerr << "Shutting down sentry" << endl;
   if (TelemetryService::exists()) {
+    cerr << "Shutting down sentry" << endl;
     auto ts = TelemetryService::get();
     ts->shutdown();
   }