Make publishable state clearer to the user

[sentry-io]

User attempted to publish a channel which had no resources, only 2 folders. This is not a valid, publishable channel. How can we make this more clear?

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Sentry Issue: STUDIO-HJC

ChannelIncompleteError: Attempted to publish a channel with an incomplete root node or no resources
  File "contentcuration/viewsets/channel.py", line 533, in publish
    channel = publish_channel(
  File "contentcuration/utils/publish.py", line 936, in publish_channel
    kolibri_temp_db = create_content_database(channel, force, user_id, force_exercises, progress_tracker=progress_tracker)
  File "contentcuration/utils/publish.py", line 138, in create_content_database
    tree_mapper = TreeMapper(
  File "contentcuration/utils/publish.py", line 206, in __init__
    raise ChannelIncompleteError("Attempted to publish a channel with an incomplete root node or no resources")

ValidationError: ["Channel is not ready to be published"]
  File "contentcuration/viewsets/channel.py", line 508, in publish_from_changes
    self.publish(
  File "contentcuration/viewsets/channel.py", line 558, in publish
    raise ValidationError("Channel is not ready to be published")