Clarify, simplify some error messages.

"cannot inherit *and* set or keep supplementary groups" is confusing,
is confusing, so split the if clause.

"usergroup inheritance without username" is inaccurate, better use
"supplementary groups" terminology.

"can only either inherit, keep or set supplementary groups;"
" tried to do two or more" can be simplified.

Bug: None
Test: Builds, unit tests pass.
Change-Id: Ib805e66dc0ca35043b9a9b73c09bae9d31ad5010
1 file changed