Incidentally, I bumped into another custom backup script just a few weeks back that also excluded backup_label. I don't know what the author was thinking when he wrote that, but it seems to be a surprisingly common mistake. Maybe it's the "label" in the filename that makes people think it's not important.
I think part of it is the name "label', and part of it is that this file is similar to and hence easily confused with the .history files, which (as far as I know) truly are there only for human information and not for system operation.
Perhaps we should improve the documentation to make it more explicit that backup_label must be included in the backup. The docs already say that, though, so I suspect that people making this mistake have not read the docs very carefully anyway.
I don't think the docs are very clear on that. They say "This file will of course be archived as a part of your backup dump file", but "will be" does not imply "must be". Elsewhere it emphasizes that the label you gave to pg_start_backup is written into the file, but doesn't really say what the file itself is there for. To me it seems to imply that the file is there for your convenience, to hold that label, and not as a critical part of the system.
Patch attached, which I hope can be back-patched. I'll also add it to commitfest-Next.
