fix error message

Was "failed to generate a key" when key generation did not fail
(it never does anymore) but the actual problem was it failed to stat
the source file, perhaps due to it being deleted while the key was being
generated.

A user reported this, in a comment I followed up on in
262400fe04, although I don't know
what they did to trigger the error message.

Annex/MetaData.hs

@@ -37,8 +37,8 @@ import Data.Time.Clock.POSIX
  -
  - Also, can generate new metadata, if configured to do so.
  -}
-genMetaData :: Key -> RawFilePath -> FileStatus -> Annex ()
-genMetaData key file status = do
+genMetaData :: Key -> RawFilePath -> Maybe POSIXTime -> Annex ()
+genMetaData key file mmtime = do
 	catKeyFileHEAD file >>= \case
 		Nothing -> noop
 		Just oldkey ->
@@ -47,11 +47,14 @@ genMetaData key file status = do
 			-- preserve any metadata already on key.
 			whenM (copyMetaData oldkey key <&&> (not <$> onlydatemeta oldkey)) $
 				warncopied
-	whenM (annexGenMetaData <$> Annex.getGitConfig) $ do
-		old <- getCurrentMetaData key
-		addMetaData key (dateMetaData mtime old)
+	whenM (annexGenMetaData <$> Annex.getGitConfig) $
+		case mmtime of
+			Just mtime -> do
+				old <- getCurrentMetaData key
+				addMetaData key $
+					dateMetaData (posixSecondsToUTCTime mtime) old
+			Nothing -> noop
   where
-	mtime = posixSecondsToUTCTime $ realToFrac $ modificationTime status
 	warncopied = warning $ 
 		"Copied metadata from old version of " ++ fromRawFilePath file ++ " to new version. " ++ 
 		"If you don't want this copied metadata, run: git annex metadata --remove-all " ++ fromRawFilePath file