Show without actors causes uncaught exception

When processing <http://thetvdb.com/?tab=series&id=385762&lid=7> an uncaught exception is raised due to there being no actors page for the series. 
Relvant part of the trace is:
```
.../tvdb_api.py", line 852, in _loadUrl
    raise tvdb_error("%s" % error)
tvdb_api.tvdb_error: not found
```