Misleading error message when target directory does not exist

[benberryallwood]

install.sh checks that $TARGET is writable, and if not it prompts the user to rerun the script using sudo.

On a clean macOS install, /usr/local/bin does not exist so the user gets this error message even when run using sudo.

[benberryallwood]

This would be fixed indirectly by #161 which adds an attempt to create the target directory