Add a few more log messages around session manager exit
Add a few more log messages around session manager exit

Now prints a message when starting, not just when exiting.

Also added messages as to why session_manager is exiting.

BUG=chromium-os:11474
TEST=Run on seaboard, check that new logging is output

Change-Id: I74b219bd56a02e5a0cd003b050bdab29385fc056

Review URL: http://codereview.chromium.org/6349113

chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager