Display a message that there are no connected printers when trying to update firmware

Contributes to CURA-274

plugins/USBPrinting/USBPrinterManager.py

@@ -95,6 +95,10 @@ class USBPrinterManager(QObject, SignalEmitter, OutputDevicePlugin, Extension):
 
     @pyqtSlot()
     def updateAllFirmware(self):
+        if not self._printer_connections:
+            Message("Cannot update firmware, there were no connected printers found.").show()
+            return
+
         self.spawnFirmwareInterface("")
         for printer_connection in self._printer_connections:
             try: