pkg-merge-upstream-to-downstreams: Fail on conflicts and when subcommands fail to parse arguments

Report the failure in the exit code if we were unable to merge some branches.

This also deals with other random errors that may occur and that at the moment are silently ignored.

Also if a subcommand exits due to an error when parsing its arguments, which is traditionally reported with exit code 2, it is not appropriate to consider it like a normal merge conflict, but we should arguably exit with an error as well.