Code review comment for lp:~al-maisan/bzr-builddeb/pristine-tar

Hello there!

This branch fixes the following kind of failure (bug #417153):

{{{
$ bzr branch lp:ubuntu/gwibber; cd gwibber; bzr bd -S
Branched 11 revision(s).
Building using working tree
Looking for a way to retrieve the upstream tarball
Using pristine-tar to reconstruct the needed tarball.
tar: gwibber-2.0.0~bzr396/.bzrignore: 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.NQspZVOeeE/recreatetarball --owner 0 --group 0 --numeric-owner -C /tmp/pristine-tar.NQspZVOeeE/workdir --no-recursion --mode 0644 --files-from /tmp/pristine-tar.NQspZVOeeE/manifest
bzr: ERROR: There was an error using pristine-tar: Generating tar from delta failed: None.
}}}

The "pristine-tar gentar" failure is now detected and an appropriate exception
(PackageVersionNotPresent) is raised to the caller so it can try to obtain the
required upstream tar ball from the next package source.

There are also two small "drive-by" fixes:

    - format string fix in MissingChangelogError
    - removal of some "dead code" in import_dsc.py

NB: this is only the first part of the "pristine-tar" problem solution. A
forthcoming branch will make sure that the "pristine-tar gendelta" command
does not produce deltas with files missing from the unpacked source tree
(thus "obscuring" future "pristine-tar gentar" problems).

Please have a look and let me know what you think.