[booth] pcs booth setup shall not jump to report "No such file or directory: '/etc/booth/booth.key'" when /etc/booth doesn't exist

[jnpkrn]

The point is that lack of /etc/booth directory signifies that, with most
likelihood, booth is just not installed (pcs apparently refrains from
creating that directory if it do not exist), as opposed to other problems
that may arise when saving that file.

Also, with Fedora:

# dnf repoquery -f /etc/booth
> booth-core-0:1.0-3.f2d38ce.git.fc31.2.x86_64

# dnf repoquery -f /etc/booth/booth.key | grep -v '^$' || echo --nobody--
> --nobody--

making deciphering and fixing of

Error: Unable to write Booth key '/etc/booth/booth.key':
  No such file or directory: '/etc/booth/booth.key'

slightly more challenging.

If /etc/booth directory is missing, it shall rather customize the
wording:

Configuration directory for booth, /etc/booth, is missing.
Is booth installed?

[jnpkrn]

Ditto pcs booth pull.

[tomjelinek]

Ack, tracked at https://bugzilla.redhat.com/show_bug.cgi?id=1791670