more precise message when image is not found

            for repo in self.repos:

                lv = max(lv, repo.last(name))

            if lv < 0:

                raise Exception("Unable to find last version of %s" % name)

                raise Exception("Unable to find image %s" % name)

            version = lv

        # search image in repos

        for repo in self.repos:

            if repo.has(name, version):

                return repo.get(name, version), repo

        raise Exception("Unable to find %s v%s" % (name, version))

        raise Exception("Unable to find image %s v%s" % (name, version))

    def show(self, verbose=False):

        '''