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.

Annex/Path.hs

@@ -17,9 +17,10 @@ import System.Environment
  - 
  - getExecutablePath is available since ghc 7.4.2. On OSs it supports
  - well, it returns the complete path to the program. But, on other OSs,
- - it might return just the basename.
+ - it might return just the basename. Fall back to reading the programFile,
+ - or searching for the command name in PATH.
  -}
-programPath :: IO (Maybe FilePath)
+programPath :: IO FilePath
 programPath = do
 #if MIN_VERSION_base(4,6,0)
 	exe <- getExecutablePath
@@ -29,6 +30,4 @@ programPath = do
 #else
 	p <- readProgramFile
 #endif
-	-- In case readProgramFile returned just the command name,
-	-- fall back to finding it in PATH.
-	searchPath p
+	maybe cannotFindProgram return =<< searchPath p

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

Assistant/Upgrade.hs

@@ -288,11 +288,8 @@ removeEmptyRecursive dir = do
 {- This is a file that the UpgradeWatcher can watch for modifications to
  - detect when git-annex has been upgraded.
  -}
-upgradeFlagFile :: IO (Maybe FilePath)
-upgradeFlagFile = ifM usingDistribution
-	( Just <$> programFile
-	, programPath
-	)
+upgradeFlagFile :: IO FilePath
+upgradeFlagFile = programPath
 
 {- Sanity check to see if an upgrade is complete and the program is ready
  - to be run. -}
@@ -303,10 +300,7 @@ upgradeSanityCheck = ifM usingDistribution
 		-- Ensure that the program is present, and has no writers,
 		-- and can be run. This should handle distribution
 		-- upgrades, manual upgrades, etc.
-		v <- programPath
-		case v of
-			Nothing -> return False
-			Just program -> do
+		program <- programPath
 		untilM (doesFileExist program <&&> nowriter program) $
 			threadDelaySeconds (Seconds 60)
 		boolSystem program [Param "version"]

Config/Files.hs

@@ -62,8 +62,13 @@ readProgramFile = do
 		( return p
 		, ifM (inPath cmd)
 			( return cmd
-			, error $ "cannot find git-annex program in PATH or in the location listed in " ++ programfile
+			, cannotFindProgram
 			)
 		)
   where
 	cmd = "git-annex"
+
+cannotFindProgram :: IO a
+cannotFindProgram = do
+	f <- programFile
+	error $ "cannot find git-annex program in PATH or in the location listed in " ++ f