Invalid config file/directory path should not be silently ignored

[mgorny]

Describe the bug
If a path other than an existing regular file is passed to --conf, or other than an existing regular directory to --confdir, dracut just silently ignores it and uses the default path.

Distribution used
Gentoo

Dracut version
051 (the relevant code is unchanged in git master)

Init system
n/a

To Reproduce
E.g. dracut --conf /dev/null --confdir /dev/null

Expected behavior
I expected dracut to either use the paths I requested (i.e. effectively use empty config) or throw an error.

Additional context
n/a