[nebula] Fix stale session issues

When Nebula isn't accessed for a while, the Zype access token stored on
the Nebula backend expires. It is then no longer returned by the user
endpoint.
The Nebula frontend has the same issue and keeps polling for the Zype
token in this case.
This isn't implemented in this extractor yet, but at least a specific
error message now prints some helpful advice.

youtube_dl/extractor/nebula.py

@@ -141,7 +141,8 @@ class NebulaIE(InfoExtractor):
         # option #4: nebula token via --videopassword
         if not nebula_token:
             nebula_token = self._downloader.params.get('videopassword')
-            if nebula_token: self.to_screen('Authenticating to Nebula with token from --videopassword')
+            if nebula_token:
+                self.to_screen('Authenticating to Nebula with token from --videopassword')
 
         if not nebula_token:
             raise ExtractorError('Nebula requires an account with an active subscription. '
@@ -228,6 +229,8 @@ class NebulaIE(InfoExtractor):
         user_object = self._call_nebula_api('/auth/user/', video_id, nebula_token, note='Retrieving Zype access token')
         access_token = try_get(user_object, lambda x: x['zype_auth_info']['access_token'], compat_str)
         if not access_token:
+            if try_get(user_object, lambda x: x['is_subscribed'], bool):
+                raise ExtractorError('Unable to extract Zype access token from Nebula API authentication endpoint, please try loading an arbitrary video in a browser with this account to ''prime'' it for video downloading')
             raise ExtractorError('Unable to extract Zype access token from Nebula API authentication endpoint')
         return access_token