[booth] `pcs constraint ticket add` does not care whether the ticket indeed exists

Either `pcs constraint ticket add` or equivalent (see #227), for which
it would be particularly critical whether:

1. booth formation is up and running

2. when 1. doesn't hold but default booth configuration file still exists

3. whether the ticket is already defined by the time is being relied
upon

When 1. or 2. holds while 3. doesn't:

- for `pcs constraint ticket add`, this shall be at least of a warning
severity (non-blocking), but perhaps a blocking with error would be
in order (unless overridden with `--force` or equivalent).

- for in-one-go shortcut, it shall be the latter right away

Similarly as with #222, user could be redirected to where the
ticket configuration can be carried (and in dreams, it would
interactively start such a process in a wizard-like manner
right away).