[automerger skipped] clean_header: Fix error handling for no-such-file case
am: 5ca5d4aa1d -s ours
am skip reason: change_id I5f717dd1a4388f598f0fd4bfd5e6129017de9095 with SHA1 6d6b4cedd1 is in history

Change-Id: I3edfd4501b8355af92bacdeb3fb2a1dbae138eb3