Fixed exit() to avoid error NameError: name 'exit' is not defined

[jbnjohnathan]

When the script fails due to Unknown message received, it will crash due to exit() not being properly defined.
Changed exit() to sys.exit()

Connecting with Corvette over serial port /dev/cu.usbserial-210...
RESET RAK11720 BOARD!!!
Sending Hello.
Received response for Hello
Received Unknown Message
msgType =  0x7720
Length =  0x7469
Traceback (most recent call last):
  File "uart_wired_update.py", line 353, in <module>
['0x54', '0x65', '0x73', '0x74', '0x20', '0x77', '0x69', '0x74', '0x68', '0x20', '0x48', '0x49', '0x47', '0x48', '0x20', '0x61', '0x6e', '0x64', '0x20', '0x4c', '0x4f', '0x57', '0xd', '0xa', '0x53', '0x65', '0x74', '0x74', '0x69', '0x6e', '0x67', '0x20', '0x74', '0x6f', '0x20', '0x48', '0x49', '0x47', '0x48', '0x2c', '0x20', '0x61', '0x6e', '0x64', '0x20', '0x77', '0x61', '0x69', '0x74', '0x69', '0x6e', '0x67', '0x20', '0x35', '0x20', '0x73', '0x65', '0x63', '0x6f', '0x6e', '0x64', '0x73', '0xd', '0xa', '0x53', '0x65', '0x74', '0x74', '0x69', '0x6e', '0x67', '0x20', '0x74', '0x6f', '0x20', '0x4c', '0x4f', '0x57', '0x2c', '0x20', '0x61', '0x6e', '0x64', '0x20', '0x77', '0x61', '0x69', '0x74']
!!!Wired Upgrade Unsuccessful!!!....Terminating the script
  File "uart_wired_update.py", line 42, in main
  File "uart_wired_update.py", line 185, in connect_device
NameError: name 'exit' is not defined
[20556] Failed to execute script 'uart_wired_update' due to unhandled exception!
Failed to burn bootloader: uploading error: exit status 1