Remove checking for missing wiki folders

Thus undoing 744245ea3249446fd15f504190efb1d828935d01 because of #1002
Coloured warnings and errors under Node.js
Add warning when serving a non-existent wiki folder

The warning will be yellow to make it noticeable
Report command errors through $tw.utils.error

So that we get the new colourisation
Use titles when dragging links to non-TiddlyWiki destinations

Handily, this means that you can drag tiddler links from the sidebar
into the text editor, making it easier to create links and
transclusions.