Show error when assets folder is missing

[thetuningspoon]

Short description of the enhancement

If the /site/assets/ folder is missing, the site will malfunction but no error is shown. The back end will often be unaccessible but the front end may still partially work. Throwing a WireException if the assets folder cannot be located would make it clear that the assets folder is missing

Why would the enhancement be useful to users?

When developers are working on a site locally, sometimes they will forget to add the /site/assets/ folder since it is generally not stored in git repos. This can lead to a lot of wasted time trying to track down why the site is behaving strangely.