dsdb-ridalloc: Rework ridalloc to return error strings where RID allocation fails

dsdb-ridalloc: Rework ridalloc to return error strings where RID allocation fails

We now also only poke the RID manager once per request.

This may help track down why RID allocation can fail, as while we
never wait for the RID set to be created/updated, it may be the only
clue the admin gets as to why the async allocations were failing.

Andrew Bartlett

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>