b2: fix failed to create file system with application key limited to a prefix

Before this change, if an application key limited to a prefix 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 "b2:bucket/path/":
        failed to HEAD for download: Unknown 401  (401 unknown)

This change assumes any errors on HEAD will make rclone assume the
object does not exist and the path is referring to a directory.

See: https://forum.rclone.org/t/b2-error-on-application-key-limited-to-a-prefix/22159/

backend/b2/b2.go

@@ -479,12 +479,9 @@ 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 {
-				// File doesn't exist so return old f
-				f.setRoot(oldRoot)
-				return f, nil
-			}
-			return nil, err
+			// File doesn't exist 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