Warning if source is missing

[euoia]

Is your feature request related to a problem? Please describe.
Hi, I just spent a little while trying to figure out why my UltiSnips snippet sources were not showing up in the completions menu. In the end, I realised it's because I made a typo and wrote snippets when the actual source should be snippet. I think the confusion might be partly due to steelsojka/completion-buffers using buffers instead of buffer.

Describe the solution you'd like
It would be nice if something could be added to :messages if you try to use a source that's not available, if possible.

Or if we could have consistency across the source names, either use plural or single across the board.

I don't know whether adding Lua examples for the configuration might help too, since I was unfamiliar with the Lua syntax I wondered for a while whether I might be doing something wrong.

It might be nice to have some commands to check which sources are available, or which sources are used, perhaps added the troubleshooting Wiki page.

Obviously a silly mistake on my part, just trying to make it a bit harder for others to make in future.