Add an error message to staged install session.

The enum itself might be a bit opaque when trying to debug what failed
during the staged session flow. Add an error message field so that the
caller can learn more about the failure, and also log this message on
logcat.

Bug: 118865310
Bug: 123360647
Test: atest PackageInstallerSessionTest; verified that error message is
written to install_sessions.xml and in logcat.

Change-Id: I45c8633f36d490c17c751a9037b90d3fd413c506