OSDB returns different types sometimes

When searching subtitles, OSDB's XMLRPC response is not always of the
same type, for example when you search for a file hash:
  - if it matches anything, we get an array of subtitle structures,
  - if it does not, we get a boolean set to false.

One way of handling this is to use a interface{} and inspect the pointed
values to determine how to cast them. Or we could just look at the error
code and do something hackish when it points to a type error... :/