[Trakt] Helpful message on error 420 (limit exceeded, e.g. list is full)

[UweTrottmann]

E.g. when adding a movie to the watchlist, but the watchlist is full display an error message that indicates the list is full instead of the generic error message to try again later.

trakt/api-help#350

https://trakt.docs.apiary.io/#introduction/status-codes

Also add message for locked user accounts while at it. #763

[UweTrottmann]

When uploading data after a sign-in the movie watchlist can also be full. Add a Sync error message for that as well.