fix: missing plugin parent dir causes unrecoverable failure

[FedeAbella]

If the plugin folder defined in the config contains parents that don't exists (e.g.: my plugin folder is ./.sf/sf_cache, and a brand new project doesn't create the .sf folder), creating the plugin folder fails. The error message was not shown, and in my case, it even prevented the lsps from loading (I'm guessing it must be something to do with the BufEnter autocmd).

This PR makes it so:

• The call to mkdir also creates parent directories if needed (no reason not to)
• The call to create the dir is protected, and the error is shown if it fails, but doesn't cause complete failure

[xixiaofinland]

Thanks again!