also avoid crashing in most circumstances if unable to determine the username

Mostly the username is only used for the git committer or other display
purposes, and we can just fall back to a dummy value in these cases.

The only remaining place where an error is thrown is when starting local
pairing, which needs the username to be known.

Annex/Environment.hs

@@ -33,7 +33,7 @@ checkEnvironment = do
 
 checkEnvironmentIO :: IO ()
 checkEnvironmentIO = whenM (isNothing <$> myUserGecos) $ do
-	username <- myUserName
+	username <- either (const "unknown") id <$> myUserName
 	ensureEnv "GIT_AUTHOR_NAME" username
 	ensureEnv "GIT_COMMITTER_NAME" username
   where
@@ -52,7 +52,7 @@ ensureCommit :: Annex a -> Annex a
 ensureCommit a = either retry return =<< tryNonAsync a 
   where
 	retry _ = do
-		name <- liftIO myUserName
+		name <- liftIO $ either (const "unknown") id <$> myUserName
 		setConfig (ConfigKey "user.name") name
 		setConfig (ConfigKey "user.email") name
 		a