core: placeholder handler revert task on failure

CreateImagePlaceholderTaskHandler:
endWithFailure() should revert task (unlocks the image)
since beforeTask() locks the image.

Change-Id: I266792d30ce4a2743a19db2b9776a3aea2eedfed
Bug-Url: https://bugzilla.redhat.com/907167
Signed-off-by: Daniel Erez <derez@redhat.com>