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>
could be, but it should start with 'Error ENOENT' so unless we change the error outputting of the ceph tool (where I think the parsing happens, and which will break most if not all the tests on this file) it's irrelevant whether we test for this string or the string you pointed out.  In a way, it's fairly more probable that the string from filesystem_command() is subjected to change than the 'Error ENOENT' part.  Keeping 'Error ENOENT' will safeguard us against the test erroring out due to a string that changed, when we only want to test is that the command fails with an ENOENT.