Prints helpful message when schema.json does not exist, as otherwise
the "file" will be mounted as an empty directory inside the
spellchecker container.
prior to this change, the EMQX spellcheck dictionary
is baked into the docker image, which makes adding
new words to the dict more complicated long process:
you'd have to send a separate PR to the docker image
repo and tag a new docker image and update the dokcer
iamge tag in emqx.git
Ivan Dyachkov