[enhancement] Print better error in the case of ownership issues

[chiraag-nataraj]

If I use private-bin to whitelist a binary in /usr/bin which happens to have the wrong owner (for example, my user), firejail prints a very opaque and unhelpful error. The context this came up in is protonmail-bridge, which (for some godforsaken reason) installed with my user as the owner (going to report that as a bug to them). This meant that private-bin failed (as it should), but the error message was unhelpful in figuring out what the problem was.

[rusty-snake]

If we want to print better error messages, we can also fix this:

$ firejail --name  # Invalid because it need =NAME
Error: invalid --name command line option
$ firejail --doesnt_exists # Invalid because it doesn't exists
Error: invalid --doesnt_exists command line option

[rusty-snake]

or this (--quiet and --debug):

$ firejail --quiet --debug XXX # or firejail --debug --quiet
Error: invalid --debug command line option

[netblue30]

Closed for now, any other problem go to #4275