convert fail to error

Using fail here causes a "user error" exception to be thrown, which implies
the user is at fault in its wording, which is incorrect.

Also audited for other uses of fail in git-annex; the others are in monadic
contexts where fail may not throw an exception, and involve user input, so
kept them as-is.

Utility/Process.hs

@@ -133,7 +133,7 @@ forceSuccessProcess p pid = do
 	code <- waitForProcess pid
 	case code of
 		ExitSuccess -> return ()
-		ExitFailure n -> fail $ showCmd p ++ " exited " ++ show n
+		ExitFailure n -> error $ showCmd p ++ " exited " ++ show n
 
 -- | Waits for a ProcessHandle and returns True if it exited successfully.
 -- Note that using this with createProcessChecked will throw away