mon: MDSMonitor: proper error output if pool DNE on 'add_data_pool'

Fixes: #9852

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
mon: MDSMonitor: have 'filesystem_command' return int instead of bool

Makes for readable, expectable, prettier code.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>