lua: getpwall/getgrall error handling - follow-up fixes

 - Add the forgotten errno(0) to getgrall.
 - Throw errors from getgrall/getpwall instead of returning nil in
   case the underlying system function fails.
 - Fix the error message in getgr.
 - Remove pointless and confusing asterisk sign from error messages.
 - Do not hide a stack frame on error.

Follow-up efccac69 ("lua: fix error handling in getpwall and
getgrall").