Fix error reporting in cp_commitChanges

Only trys to commit if necessary, and reports errors
if commiting fails. RemoveFileIfExists returns true on
success.

Test: vdc checkpoint startCheckpoint, reboot, and then
      vdc checkpoint commitChanges
Bug: 111020314

Change-Id: Ie1b3e49beb3ca04f2881fcc595882c607368b477
1 file changed