Document mender-artifact dependency on 'debugfs'

mender-artifact requires:

• install of e2fsprogs, which contains `debugfs`
• debugfs must be in PATH

Above is needed for the "cp", "cat" and "install" commands and is not documented anywhere.

Don Cross, also reported:

It turns out I do have a debugfs executable on my system (Debian Stretch 9.4.0), only it is under /sbin, which is not in my normal user's PATH

Acceptance criteria:

• mender-artifact checks if debugfs is available when required (e.g. cp, cat, install) and exits with a descriptive error message if not
• the error message includes the name of the required binarie(s) and typical package(s) they would be found in