No prompt: dvc get for an existing file

[dmpetrov]

$ touch small.txt
$ dvc get https://github.com/dmpetrov/dataset1 small.txt
file 'small.txt' is going to be removed. Are you sure you want to proceed? [y/n] y

It is better to get rid of asking prompts from users - see #2498. In this case, failing is the appropriate behavior.

[skshetry]

dvc get these days fails with:

unexpected error - [Errno 17] File exists: 'small.txt'

However, the messaging could be improved.