Populate error message if apexd.markStagedSessionReady fails

This is a minimum change required to merge apexd CL in this topic,
which will unblock merging it into AOSP.

Test: atest CtsStagedInstallHostTestCases
Bug: 136548037
Bug: 122952270
Change-Id: I97fef3cb4b162568b493ed144fe270415db01ab9
Merged-In: I6fde70418990ee27c1966619badc437c9bffb697