Print error message if livedown executable can't be found

[ethagnawl]

Closes #30.

Also adds support for new livedown plugin directory structure (node_modules/.bin => node_modules/bin).

[shime]

Thanks for contributing!

Unfortunately your changes do not seem to detect the absence of the livedown executable. I use nodenv and when I delete livedown with npm uninstall -g livedown, these changes do not detect the absence properly.

I've tried implementing the changes yesterday, but for some reason the Vim's executable function does not work properly in this scenario.

[ethagnawl]

Is a local version of livedown being detected via one of the two relative node_modules paths?

I'll note that I don't totally understand what the use case for the relative paths is. On my setup it winds up looking in ~/.vim/plugged/node_modules, which seems a little counter intuitive. I just ran with what was already in place -- and added the new bin path -- in order to not break any existing setups.

[ethagnawl]

@shime Did you ever get a chance to re-review this in light of what I said above? I just tried using this plugin without the Node livedown package installed and my fork correctly reported the issue and the suggested solution.