Exits if you start with a --master=/dev/ttyXXX that doesnt exist

If you start with a master tty that doesn't exist, Mavproxy crashes out, but if you add a link that doesn't exist during runtime, it handles it fine.

This is particularly problematic for mavgateway - it means you can't just set "all" of the possible masters and go with whatever you've got connected, you HAVE to list the right one, which requires script editing.

Start by tracing MAVProxy's startup handling for the --master option and compare it with the runtime path for adding a missing link. Confirm the behavior with a nonexistent /dev/ttyXXX master; done means startup remains usable and handles the missing master consistently with a link added during runtime.

Written by the indexing model from the issue text.