Fix system server crash caused by package installer

Package installer crashes when trying to install a staged session
but is already applied or fails. Even if session validation fails,
the service should not crash the system. Also refine the error
message in terms of which session is causing the issue and how we
got to that state.

Bug: 286306412
Test: Build successfully and boot to home
Change-Id: I0de3ef888fba0c81a0d404dfdce4389a5b7bfd24