downloader: more informative errors when desired release not available
downloader: more informative errors when desired release not available

LeMeteore/stdeb,LeMeteore/stdeb,LeMeteore/stdeb,benthomasson/stdeb,pombredanne/stdeb,benthomasson/stdeb,pombredanne/stdeb,astraw/stdeb,sathieu/stdeb,sathieu/stdeb,astraw/stdeb,sathieu/stdeb,astraw/stdeb,benthomasson/stdeb,pombredanne/stdeb