[login_manager] Clean up error message logging/glib error reporting
[login_manager] Clean up error message logging/glib error reporting

BUG=none
TEST=unit tests

Change-Id: Ic88ef7e3b5ebd4a0378a2c6e031cd563dbbc001f

Review URL: http://codereview.chromium.org/6614020

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