* get/import/url: handle existing output directory
Currently if you run
```
dvc get https://github.com/iterative/example-get-started model.pkl --out .
```
it will nuke your cwd. To fix that we need to properly handle existing
directories, same as usual unix cli utilities do that.
* travis: specify python version when installing from chocolatey
Fixes#2609
* get: show error message when path is a DVC-file
Fix#2468
* exceptions: rename output for data artifact
* get: use method to indetify if file is a DVC-file
* exceptions: change GetDVCFileError message
Ruslan Kuprieiev
Mr. Outis
pawel
Ruslan Kuprieiev