Serial Monitor Tool: Display Error When Serial Port is In Use

[rcarteraz]

I noticed that if the serial port is currently in use by another program when I try to open the serial monitor tool in the Meshtastic Web-Flasher, it just doesn't open without any explanation. If possible, it would be helpful if there was some kind of error message that pops up letting me know the serial port is already taken or to check if it's being used elsewhere.