s3: fix failed to create file system with folder level permissions policy

Before this change, if folder level access permissions policy was in
use, with trailing `/` marking the folders then rclone would HEAD the
path without a trailing `/` to work out if it was a file or a folder.
This returned a permission denied error, which rclone returned to the
user.

    Failed to create file system for "s3:bucket/path/": Forbidden: Forbidden
        status code: 403, request id: XXXX, host id:

Previous to this change

53aa03cc44 s3: complete sse-c implementation

rclone would assume any errors when HEAD-ing the object implied it
didn't exist and this test would not fail.

This change reverts the functionality of the test to work as it did
before, meaning any errors on HEAD will make rclone assume the object
does not exist and the path is referring to a directory.

Fixes #4990

backend/s3/s3.go

@@ -1694,12 +1694,9 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		f.setRoot(newRoot)
 		_, err := f.NewObject(ctx, leaf)
 		if err != nil {
-			if err == fs.ErrorObjectNotFound || err == fs.ErrorNotAFile {
-				// File doesn't exist or is a directory so return old f
-				f.setRoot(oldRoot)
-				return f, nil
-			}
-			return nil, err
+			// File doesn't exist or is a directory so return old f
+			f.setRoot(oldRoot)
+			return f, nil
 		}
 		// return an error with an fs which points to the parent
 		return f, fs.ErrorIsFile