[automerger skipped] clean_header: Fix error handling for no-such-file case am: 5ca5d4aa1d -s ours
am: eff8890dbd -s ours
am skip reason: change_id I5f717dd1a4388f598f0fd4bfd5e6129017de9095 with SHA1 6d6b4cedd1 is in history

Change-Id: I63a42d3bcf1811970aed724976bc89f03956f6f1