Add proper custom errors object creations (#7387)

In scenario 1

```
- bucket/object-prefix
- bucket/object-prefix/object
```

Server responds with `XMinioParentIsObject`

In scenario 2

```
- bucket/object-prefix/object
- bucket/object-prefix
```

Server responds with `XMinioObjectExistsAsDirectory`

Fixes #6566

cmd/fs-v1-multipart.go

@@ -487,7 +487,7 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 
 	// Check if an object is present as one of the parent dir.
 	if fs.parentDirIsObject(ctx, bucket, pathutil.Dir(object)) {
-		return oi, toObjectErr(errFileAccessDenied, bucket, object)
+		return oi, toObjectErr(errFileParentIsFile, bucket, object)
 	}
 
 	if _, err := fs.statBucketDir(ctx, bucket); err != nil {