clarify loginviashell error message

When adding a new member to a project, the user needs to have the
'loginviashell' user right. The previous comment was not very clear and
could let the user performing the action he is missing that user right.

Change-Id: I78a34c17f7e7dc96b9aa1119776317421d584fc3
Merge "clarify loginviashell error message"