useradd and groupadd don't describe failure conditions

[Jeff Johnson]

The LSB specifications for useradd/groupadd are important to consider when
trying to create portable packages which need to create users and/or groups.
However, as noted in bug 1047, there is no description of failure modes/return
values, so it's hard to tell how to behave if one of these commands fails. As
perhaps the most notable example, if useradd fails, can we assume the user
already exists? should we check return values more carefully before deciding
that? "man useradd" where linux-manpages was 3.09 shows nine possible failures
for useradd with non-consecutive values in the range 1-13; groupadd shows five
in the range 2-10. Would listing those be too detailed (i.e., do we know
everyone has exactly the same useradd/groupadd)