more realistic limit on getorigtarball

from getorigtarball:

...
        # Set download of files, when the expected file size of the orig.tar.gz is larger
        # than the configured threshold.
        # XXX TODO: This size should be the 75% quartile, that is the value where 75% of all
        #           packages are smaller than that. For now, blindly assume this as 15M
        size = 15728640
...
        for dscfile in dsc['Files']:
            dscfile['size'] = int(dscfile['size'])
            if orig == dscfile['name']:
                if dscfile['size'] > size:
                        log.warning("Skipping eventual download of orig.tar.gz %s: size %d > %d" % (dscfile['name'], dscfile['size'], size))
                        return

but 15 MB is very very little IMHO, somebody just hit this with a 54 MB tarball for openjdk-7.

Also, most likely this should cause a proper rejection stating that the failure in this plugin caused it, since it seems it currently is opaque from a user POV.