snapcraft kernel: Failed to download 'ubuntu-core/edge'

snapcraft -d pull
Loaded local plugin for kernel
Preparing to pull kernel
Pulling kernel
From git://kernel.ubuntu.com/ubuntu/ubuntu-xenial
 * branch HEAD -> FETCH_HEAD
Already up-to-date.
Getting details for 'ubuntu-core/edge'
Encoding URI, headers and body to utf-8.
Downloading 'ubuntu-core/edge'
Encoding URI, headers and body to utf-8.
Traceback (most recent call last):
  File "/usr/bin/snapcraft", line 31, in <module>
    snapcraft.main.main()
  File "/usr/lib/python3/dist-packages/snapcraft/main.py", line 126, in main
    commands.load(cmd).main(argv=args['ARGS'])
  File "/usr/lib/python3/dist-packages/snapcraft/commands/pull.py", line 39, in main
    lifecycle.execute('pull', args['PART'])
  File "/usr/lib/python3/dist-packages/snapcraft/lifecycle.py", line 53, in execute
    _Executor(config).run(step, part_names)
  File "/usr/lib/python3/dist-packages/snapcraft/lifecycle.py", line 81, in run
    self._run_step(step, part, part_names, dirty, recursed)
  File "/usr/lib/python3/dist-packages/snapcraft/lifecycle.py", line 116, in _run_step
    getattr(part, step)()
  File "/usr/lib/python3/dist-packages/snapcraft/pluginhandler.py", line 270, in pull
    self.code.pull()
  File "/home/glaure/Mercurial/ubuntu-xenial/parts/plugins/x-kernel.py", line 306, in pull
    'ubuntu-core/edge', self.os_snap, config, self._target_arch['deb'])
  File "/usr/lib/python3/dist-packages/snapcraft/storeapi/_download.py", line 56, in download
    raise RuntimeError('Failed to download {!r}'.format(snap))
RuntimeError: Failed to download 'ubuntu-core/edge'

Sorry do not be confused with x-kernel.py. I copied kernel.py from /usr/lib/python3/dist-packages/snapcraft/plugins to patch the initrd decompression from gzip to xz.
Everything else is completely unchanged

My proposal to solve such issues:

    - kernel-os-snap:
      (string, optional)
      provide a local os.snap file. If provided no os.snap has to be downloaded during the pull phase.

kernel.py:

    def pull(self):
        super().pull()
        config = load_config()

        if self.options.kernel_os_snap:
            logger.info('local os image: {0}, os={1}'.format(
                self.options.kernel_os_snap, self.os_snap))
            if os.path.exists(self.options.kernel_os_snap):
                shutil.copyfile(self.options.kernel_os_snap, self.os_snap)
        else:
            logger.info('storeapi os image: channel={0}, os={1}, arch={2}'.format(
            'ubuntu-core/edge', self.os_snap, self._target_arch['deb']))

            storeapi.download(
                'ubuntu-core/edge', self.os_snap, config, self._target_arch['deb'])

https://github.com/ubuntu-core/snapcraft/pull/452