improve error message

Command/Migrate.hs

@@ -80,7 +80,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			forM_ urls $ \url ->
 				setUrlPresent newkey url
 			next $ Command.ReKey.cleanup file oldkey newkey
-		, error "failed"
+		, giveup "failed creating link from old to new key"
 		)
 	genkey Nothing = return Nothing
 	genkey (Just fm) = fm oldkey newbackend afile >>= \case

Command/ReKey.hs

@@ -68,7 +68,7 @@ perform :: FilePath -> Key -> Key -> CommandPerform
 perform file oldkey newkey = do
 	ifM (inAnnex oldkey) 
 		( unlessM (linkKey file oldkey newkey) $
-			giveup "failed"
+			giveup "failed creating link from old to new key"
 		, unlessM (Annex.getState Annex.force) $
 			giveup $ file ++ " is not available (use --force to override)"
 		)