Error 404 can actually mean your site is WORKING... Not BROKEN!
It's commonly mistaken by a lot of webmasters that error 404 means something is broken and the site must be fixed! when in reality 50% of the time errors 404 occur when a user or bot requests a URL that no longer exists or never has existed meaning a 404 return means everything is working as intended.
When 404's can hurt SEO
Generally 404's will not harm a site unless you have hundreds to thousands actually on the page that would dampen the user experience for your visitors. One way of this not helping SEO is that Google will often use on page external links to associate relevancy of content, obviously if the link is not working the page can't be rewarded. So often you may get an decrease in SEO on a page its unlikely to harm the site unless you got far too many which in your case you don't.
Fixing it for Webmaster Tools
A simple fix would to actually create a page on the same path that Google is requesting. In the header response use
GONE 410 and
no-index. This will tell Google and other search engines that this page no longer exists. You could actually use a robots.txt solution as well but the prefer methods are doing the status actually on the page that way you keep your robots and htaccess clean.
Yet another solution...
Another solution would be to address Layerslider, rather than setting the path skin you could just copy the folder to the default path
/layerslider/skins/. Then if you have a custom skin you just set it using
skin : 'myskinexample'. You could even edit the JS to make your own path the default. Since you've purchased this skin you should be able to get support from CC.