ci: check if given schema.json exists before running spellcheck

Prints helpful message when schema.json does not exist, as otherwise
the "file" will be mounted as an empty directory inside the
spellchecker container.

scripts/spellcheck/spellcheck.sh

@@ -12,6 +12,11 @@ else
     SCHEMA="$(realpath "$1")"
 fi
 
+if ! [ -f "$SCHEMA" ]; then
+  echo "Schema file $SCHEMA does not exist; did you forget to run 'make emqx{,-enterprise}' ?"
+  exit 1
+fi
+
 set +e
 docker run --rm -i --name spellcheck \
     -v "${PROJ_ROOT}"/scripts/spellcheck/dicts:/dicts \