Handle unknown movies on refresh with TMDB Backend

[NaPs]

Sometime movies are deleted on TMDB, so the identifier bound to a movie ("_tmdb_id") in the metadata db is not more valid. This is a problem while an update for the TMDB Backend. The current behavior is to stop the update and print a too vague error to the user (like "Unable to get URL").

1. This problem should not stop an update.
2. A more explicit warning should be printed to the user saying that the movies doesn't exists.
3. A mechanism on the backends API must be implemented to print this warning.