Improve error message when target directory doesn't exist

[paulRbr]

Steps to reproduce

> bump deploy inexistent/dir/ --auto-create --hub "my-hub" --token "my-token"
 ›   Error: Missing required flag:
 ›    -d, --doc DOC  Documentation public id or slug. Can be provided via BUMP_ID environment variable
 ›   See more help with --help

Expected behavior

A better error message, stating that the target directory inexistent/dir/ doesn't exist.