Show error when pressing <C-x><C-t> and there are no completions for the current word

[arp242]

Right now it simply "does nothing", which isn't very helpful. Using other <C-x>* completions give a "pattern not found" error.

Sounds easy, but :echo and :echom get override by the INSERT text; so not sure how we can do that.

See: #6 (comment)