Add usage error

dotfiles/bash/main.bashrc

     # Check if YAML file $1 is loadable
     #
     local yfile=$1
+    test -n "$yfile" || { echo "usage: ckyaml FILE.yaml"; return 2; }
     python -c 'import yaml; import sys; print(yaml.load(sys.stdin)["this"])' <"$yfile"
 }