The "gendelta" command should abort as opposed to generating broken deltas

Binary package hint: pristine-tar

When a file is missing from the unpacked source tree "pristine-tar gendelta" will happily generate a (broken) delta without as much of a warning. It should abort with an error instead.

The error can be reproduced easily:

{{{
$ apt-get source apg
Reading package lists... Done
Building dependency tree
Reading state information... Done
Need to get 90.2kB of source archives.
Get: 1 http://us.archive.ubuntu.com karmic/universe apg 2.2.3.dfsg.1-2 (dsc) [620B]
Get: 2 http://us.archive.ubuntu.com karmic/universe apg 2.2.3.dfsg.1-2 (tar) [83.6kB]
Get: 3 http://us.archive.ubuntu.com karmic/universe apg 2.2.3.dfsg.1-2 (diff) [5993B]
Fetched 90.2kB in 0s (208kB/s)
dpkg-source: info: extracting apg in apg-2.2.3.dfsg.1
dpkg-source: info: unpacking apg_2.2.3.dfsg.1.orig.tar.gz
dpkg-source: info: applying apg_2.2.3.dfsg.1-2.diff.gz
$ cd apg-2.2.3.dfsg.1/
$ rm README.CYGWIN
$ pristine-tar gendelta ../apg_2.2.3.dfsg.1.orig.tar.gz ../brokendelta
$ echo $?
0
$ rm ../apg_2.2.3.dfsg.1.orig.tar.gz
$ pristine-tar gentar ../brokendelta ../apg_2.2.3.dfsg.1.orig.tar.gz
tar: apg-2.2.3.dfsg.1/README.CYGWIN: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
pristine-tar: command failed: tar cf /tmp/pristine-tar.lLB0MOTtSg/recreatetarball --owner 0 --group 0 --numeric-owner -C /tmp/pristine-tar.lLB0MOTtSg/workdir --no-recursion --mode 0644 --files-from /tmp/pristine-tar.lLB0MOTtSg/manifest
}}}