Experimental offline mode fails with network present

[Igor Ljubuncic]

The experimental offline mode in snapcraft does not work as intended, and does not have a graceful failover.

How to reproduce:

Ubuntu 18.04 system (and also in some cases 20.04)
Run snapcraft --offline (with lxd or multipass) on any snapcraft.yaml

The run will fail with:

Sorry, an error occurred in Snapcraft:
'NoneType' object is not subscriptable
We would appreciate it if you anonymously reported this issue.

Complete trace:

Traceback (most recent call last):
  File "/snap/snapcraft/6954/bin/snapcraft", line 8, in <module>
    sys.exit(run())
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 1137, in __call__
    return self.main(*args, **kwargs)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 1062, in main
    rv = self.invoke(ctx)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 1646, in invoke
    super().invoke(ctx)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 763, in invoke
    return __callback(*args, **kwargs)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/cli/_runner.py", line 130, in run
    snap_command.invoke(ctx)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/click/core.py", line 763, in invoke
    return __callback(*args, **kwargs)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/cli/lifecycle.py", line 387, in snap
    _execute(steps.PRIME, parts=tuple(), pack_project=True, output=output, **kwargs)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/cli/lifecycle.py", line 125, in _execute
    with build_provider_class(
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_base_provider.py", line 87, in __enter__
    self.create()
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_lxd/_lxd.py", line 284, in create
    self.launch_instance()
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_base_provider.py", line 263, in launch_instance
    self._setup_snapcraft()
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_lxd/_lxd.py", line 481, in _setup_snapcraft
    super()._setup_snapcraft()
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_base_provider.py", line 508, in _setup_snapcraft
    snap_injector.apply()
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_snap.py", line 392, in apply
    snap.push_host_snap(file_pusher=self._file_pusher)
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/build_providers/_snap.py", line 141, in push_host_snap
    host_snap_repo.local_download(
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/repo/snaps.py", line 181, in local_download
    if self.has_assertions():
  File "/snap/snapcraft/6954/lib/python3.8/site-packages/snapcraft/internal/repo/snaps.py", line 152, in has_assertions
    return not self.get_local_snap_info()["revision"].startswith("x")
TypeError: 'NoneType' object is not subscriptable

Expected behavior:

1. If snapcraft --offline is run and there is network and NO cached assets exist, it should inform the user that it needs to pull the assets from the network the first time.
2. If snapcraft -offline is run and there is network and cached assets, it should run as though there is no network; and there should be no errors.
3. If snapcraft --offline is run and there is no network and no cached assets, it should inform the user that it cannot complete the task as there are no available assets that can be retrieved.
4. If snapcraft --offline is run and there is no network and but there are cached assets, it should complete normally.

Also, if snapcraft is run without offline mode (not specified) and there is no network, it should check for network status first before trying to launch the container, as the container startup will time out (unspecified) but this behavior cannot be distinguished from: a) slow network b) slow vm/container behavior where it can take a long time (minutes) for the vm/container to start.