Clarify error message when `--target` directory doesn't exist

[thislooksfun]

The current error message if the --target directory doesn't exist looks like this:

stow: --target value '<path>' is not a valid directory

I personally found this wording to be quite confusing, and I actually ended up having to search the source code for the error to find what it actually meant. The phrasing "is not a valid directory" reads to me as "this is a directory, but you're not allowed to use it", not as "the target doesn't exist".

Additionally, having the full help print out in this error case makes it harder to see the error message at all.

I propose the following changes:

1. Reword the error to be --target value '<path>' doesn't exist or is not a directory
2. Don't automatically print the help if the --target is invalid

[aspiers]

Interesting, I never thought of that ambiguity but I see where you're coming from. Your proposed changes seem reasonable and I'd welcome a PR for them. As mentioned in #124 I do personally have bigger fish to fry though.