add: improve message for overlapping outputs

[shcheklein]

Bug Report

Please provide information about your setup

Output of dvc version:

$ dvc version
DVC version: 1.6.6+79e8e4
---------------------------------
Platform: Python 3.8.5 on macOS-10.15.6-x86_64-i386-64bit
Supports: All remotes
Cache types: reflink, hardlink, symlink
Cache directory: apfs on /dev/disk1s1
Workspace directory: apfs on /dev/disk1s1
Repo: dvc, git

Reproduce

Run dvc add on a directory with some data twice and get something like:

(.env) √ Projects/test-dvc % dvc add data
Stage is cached, skipping

Problem

stage term is not applicable here. This message should be fixed/changed. #5385

Bonus

Error message on the overlapping data should be avoiding term output. Instead we should be using directory or file. Also should we recommend dvc add data instead of commit (might be easier for users, no need to learn a new command).

(.env) √ Projects/test-dvc % dvc add data/file3
Adding...
ERROR: Cannot add 'data/file3', because it is overlapping with other DVC tracked output: 'data'.
To include 'data/file3' in 'data', run 'dvc commit data.dvc'

[gsvolt]

@shcheklein - may I work on this issue?

[shcheklein]

@gsvolt yes! absolutely, feel free to take it! thanks 🙏

[DiPaolo]

Is it fine to use a message similar I've tried to use at #5236? Something like this:
WARNING 'data' is already under DVC control

If it's okay, I will fix the issue soon.

[efiop]

Repurposing for the "bonus"

[skshetry]

Note that the message these days says something like following:

ERROR: bad DVC file name 'data/file3.dvc' is git-ignored.