@NonNull
    static Single<Boolean> addToWatchlist(@NonNull final Geocache cache) {
        return addToOrRemoveFromWatchlist(cache, true);
    }
Adds the cache to the watchlist of the user.

@param cache the cache to add
@return {@code false} if an error occurred, {@code true} otherwise
main/src/main/java/cgeo/geocaching/connector/gc/GCParser.java