Add workaround for missing actors page

[dbr]

Accessing the actors API endpoint for a show with no actors added will throw a tvdb_error as if the show is missing.

This is technically a bug with TVDB - it should return an empty list of actors, not 404.. but we can easily workaround this in the API wrapper (a non-existent show will error before that point)

[mueslo]

This ist still critical. The further down this has to be mitigated, the uglier it becomes, e.g. the only way to work around this is to have two tvdb_api.Tvdb instances, one of which does not query for actors, and have this one called if the first one errors out. Hideous.

[mueslo]

Any update? I heard back from TheTVDB and they will not be fixing it on their end (only in their next API version...)