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

Pull request also has two minor patches: one adding proper docs to a function
added in a previous pull request for #9794, another refactoring a function to
make it a bit saner according to the changes introduced when fixing #9794.

Fixes: #9852

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
Shouldn't this be the `ss << "pool '" << poolname << "' does not exist";` message that's set in filesystem_command?