make programPath return FilePath not Maybe FilePath

Looking at the few current callers, it's ok to have programPath throw an
exception, in the unusual case where it cannot find git-annex.

Assistant/Threads/UpgradeWatcher.hs

@@ -36,8 +36,7 @@ upgradeWatcherThread urlrenderer = namedThread "UpgradeWatcher" $ do
 		showSuccessfulUpgrade urlrenderer
 	go =<< liftIO upgradeFlagFile
   where
-	go Nothing = debug [ "cannot determine program path" ]
-	go (Just flagfile) = do
+	go flagfile = do
 		mvar <- liftIO $ newMVar InStartupScan
 		changed <- Just <$> asIO2 (changedFile urlrenderer mvar flagfile)
 		let hooks = mkWatchHooks