mon/OSDMonitor: further improve prepare_command_pool_set E2BIG error message

mon/OSDMonitor: further improve prepare_command_pool_set E2BIG error message

d2c0fe9b5319a4404965c40ec92e291802ef30f6 improved this error message,
but it can be improved further by suggesting that the pg_num be increased in
smaller increments.

This commit is not cherry-picked from master because 4d3407ba93b2c15dd86aa7f97bd9f29c3508c60e
(merged for Nautilus) removed the error message in question.

Fixes: http://tracker.ceph.com/issues/39353
Signed-off-by: Nathan Cutler <ncutler@suse.com>
(cherry picked from commit 7b78bad91605da4d7c9f009803323f7fbc930497)