Mavproxy does not wait/fail for device to be available

Latest version of mavproxy assume connected/device available even if it's not..

I've always been running mavproxy as following for example:
while sleep 0.1; do mavproxy --mav20 --master /dev/serial/by-id/usb-Hex_ProfiCNC_CubeOrange_*-if00 --out udp:127.0.0.1:14550 ; done

Which allowed mavproxy to fail when the device is not available and connect once it's there..
On latest version, it does not fail with "device not found" but opens interactive the shell as if the device is there and then fails with any further command with error ERROR in command []: 'NoneType' object has no attribute 'mav'

It has been working fine on version 1.8.46

Reproduce the issue with the reported mavproxy command and an unavailable /dev/serial/by-id device, then compare the latest version with 1.8.46. Trace the mavproxy startup path for an unavailable master. Done means it fails with a device-not-found error instead of opening an interactive shell and later raising the NoneType error.

Written by the indexing model from the issue text.