Synchronously decode as much of the telemetry stream as possible before launching the gui, so that errors can be emitted before creating the window.

Copied from Perforce
 Change: 194303

mps/tool/monitor

@@ -599,10 +599,14 @@ def main():
 
     with open(args.telemetry, 'rb') as telemetry_file:
         event_queue = queue.Queue()
+        model = Model(event_queue)
+        decoder = telemetry_decoder(telemetry_file.read)
+        for batch in decoder():
+            event_queue.put(batch)
+            model.update()
         stop = threading.Event()
 
         def decoder_thread():
-            decoder = telemetry_decoder(telemetry_file.read)
             while not stop.isSet():
                 for batch in decoder():
                     if stop.isSet():
@@ -611,7 +615,6 @@ def main():
 
         thread = threading.Thread(target=decoder_thread)
         thread.start()
-        model = Model(event_queue)
         qapp = QtWidgets.QApplication([])
         app = ApplicationWindow(model, args.telemetry)
         app.show()