cli: what should --quiet mode do?

[skshetry]

Before we were using logger and were suppressing methods in --quiet mode. So, dvc metrics/params diff/show tables and --show-json didn't use to print anything at all. This has now been changed to always display the table, and we have agreement that --show-json should follow the same.

There's still an issue about what we should do with fatal flaws or strong warnings. We could take an example of a failed/errored out script in dvc repro -q. At the moment, we don't print anything at all and simply exit with code 1.

So, let's have a guideline around it as:

1. If the command's (or, the option) that sole purpose is to display the information, let show it in any case.
2. If the information is very important to the user, print warnings and error messages to the stderr even in --quiet mode wherever appropriate.

If someone disagrees with this proposal, please feel free to voice your concerns below. Cheers!