Improve dvc pull recursive option UX

[shcheklein]

From discord: https://discordapp.com/channels/485586884165107732/519377613110902784/553649896264040505

And the follow up:

yes! I think so. basically -r is just a way to change list of targets specified
and in case of dvc pull targets are DVC files, not data files
so, when you specified it it was probably looking for DVC files in the directory. I'm not sure why did it write anything at all though.
Also, I would write an error message/warning if -R is specified and target is not directory
so there is def some space for improvement

[shcheklein]

I would check and implement the same warnings if needed for other commands that accept DVC files and sometimes directories (metrics show -R?). At least in case when -a or -T are not specified it should print a warning if path is not a directory?

[skshetry]

Closing due to lack of activity. It's a nice to have, but these days we support pulling files just by path: dvc pull data/subdir, etc.