Refactor: libpe_status: Best practices for pcmk__unpack_fencing_topology

Rename to pcmk__validate_fencing_topology() and drop the scheduler
argument, since we're not truly unpacking anything. Throw an error
instead of a warning upon configuration error. Improve error messages.
Check separately for unparsable integer.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>