Return error on not found from ListFn

This changes `ListFn`'s implementation so that if it encounters a not
found error, instead of sending a fatal error to log, it coordinates the
return of the error between checker goroutines and sends it back to the
caller.

The main impetus here is that it allows an external program compiling
against rclone as a package to handle a not found, where it currently it
cannot.

This does change error output on a not found a little bit, we go from
this:

    2017/01/09 21:14:03 directory not found

To this:

    2017/01/09 21:13:44 Failed to ls: directory not found

fs/operations.go

@@ -654,7 +654,10 @@ func ListFn(f Fs, fn func(Object)) error {
 			for {
 				o, err := list.GetObject()
 				if err != nil {
-					log.Fatal(err)
+					// The error will be persisted within the Lister object and
+					// we'll get an opportunity to return it as we leave this
+					// function.
+					return
 				}
 				// check if we are finished
 				if o == nil {
@@ -667,7 +670,7 @@ func ListFn(f Fs, fn func(Object)) error {
 		}()
 	}
 	wg.Wait()
-	return nil
+	return list.Error()
 }
 
 // mutex for synchronized output