Fix for errors caused when searching tvrage for release estimations

Just a quick fix, so that Flexget won't just close when it happens. 

This is a fix for tickets #2276 and #2222 (there could be more, though)

It can  be fixed better, for example by adjusting the series name in user's configurations to match that of TVrage's. However if users makes series filter from trakt_list for example, it's not possible to change series' name.

I tried to experimentally add a way of directly telling ragetv ID for complicated shows (for example trakt_list could pass this on too so it would be handy to have), but problems I faced were:

- Lot of spaghetti-passing of task instances through lot of methods from discover.py up until flexget's api_tvrage.py
- 3rd party python-tvrage does not support looking up show by telling it tvrage ID directly - this could be solved by either overriding their method, or merging their API to flexget's directly (could be beneficial, because there hasn't been an update for python-tvrage in a while, so it could be abandoned)