change retrieveExport and getKey to throw exception

retrieveExport is part of ongoing transition to make remote methods
throw exceptions, rather than silently hide them.

getKey very rarely fails, and when it does it's always for the same reason
(user configured annex.backend to url for some reason). So, this will
avoid dealing with Nothing everywhere it's used.

This commit was sponsored by Ilya Shlyakhter on Patreon.

Command/Migrate.hs

@@ -89,10 +89,8 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			, contentLocation = content
 			, inodeCache = Nothing
 			}
-		v <- genKey source nullMeterUpdate (Just newbackend)
-		return $ case v of
-			Just (newkey, _) -> Just (newkey, False)
-			_ -> Nothing
+		newkey <- fst <$> genKey source nullMeterUpdate (Just newbackend)
+		return $ Just (newkey, False)
 	genkey (Just fm) = fm oldkey newbackend afile >>= \case
 		Just newkey -> return (Just (newkey, True))
 		Nothing -> genkey Nothing