myst start crashes when `myst.yml` is corrupt

[parmentelat]

I have vs-code in auto-save mode whenever I mess with my myst.yml config file, and auto-save decides to save an unfinished, myst start crashes abruptly in these situations I find myself double checking all the time for myst start to be still alive

not that I change that file very often, but well, it'd help if in these situations myst start was just to issue an error but kept on watching for changes

for the record mostly, nothing too serious either at this point, obviously