plots modify minor issues

[dmpetrov]

$ dvc -V
1.0.0a6+4cfc2d

1. Misleading message (log: show modifying if entry already exists in dvcfile #3978)

$ dvc plots modify plots.csv --unset y title
Adding stage 'trainme' to 'dvc.yaml'

It should be Modifying stage 'trainme' to 'dvc.yaml'

2. --unset should handle errors properly

$ dvc plots modify plots.csv --unset y_fake_param
Adding stage 'trainme' to 'dvc.yaml'
$ echo $?
0

It supposed to fail with proper message. #4044

3. No template file check before setting (plots modify: complain if template does not exist #3995)

$ dvc plots modify -t confusion1 plots.csv
Adding stage 'trainme' to 'dvc.yaml'

While there is no template confusion1. It should fail with a proper error message.

4. not easy to understand --unset syntax
   It supposed to be in the end, right? Initially I tried dvc plots modify --unset y plots.csv that didn't work. Same as in plots: diff: targets and revisions behaviour #3962

5. message

$ dvc plots -h
    modify            Modify plot props associated with a target file.

target file info is not needed. It can be Modify plot properties.

6. message (plots: fix help message #3983)

$ dvc plots modify -h

props term is often used. It was not clear what foes it mean. We should probably clarify.