Propagate error while scanning apk-in-apex to session object

By storing the error message on session object, we avoid searching the
logcat (which is not always available) for information on why
apk-in-apex scanning failed.

Bug: 178192690
Test: atest ApexManagerTest
Test: atest StagedInstallTest#testApexFailsToInstallIfApkInApexFailsToScan
Change-Id: I1be52da133ea534ba103f5fbdea5b8138ef67ef6