First, we write the key a whole lot less. We write it on creation, then
never again.
Second, it's in a file we control very closely. Instead of blindly
regenerating the key if the target file generates an error on read,
we block startup unless the error is 'ENOENT' - the file isn't there
at all.
This still allows for the key.txt file to be deleted or corrupted
somehow, but it should be a lot less common than the high-traffic
config.json used for window location and media permissions.
