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/api-errors.go

@@ -180,6 +180,7 @@ const (
 	// Minio extended errors.
 	ErrReadQuorum
 	ErrWriteQuorum
+	ErrParentIsObject
 	ErrStorageFull
 	ErrRequestBodyParse
 	ErrObjectExistsAsDirectory
@@ -858,6 +859,11 @@ var errorCodes = errorCodeMap{
 		Description:    "Storage backend has reached its minimum free disk threshold. Please delete a few objects to proceed.",
 		HTTPStatusCode: http.StatusInsufficientStorage,
 	},
+	ErrParentIsObject: {
+		Code:           "XMinioParentIsObject",
+		Description:    "Object-prefix is already an object, please choose a different object-prefix name.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrRequestBodyParse: {
 		Code:           "XMinioRequestBodyParse",
 		Description:    "The request body failed to parse.",
@@ -1561,6 +1567,8 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 		apiErr = ErrObjectExistsAsDirectory
 	case PrefixAccessDenied:
 		apiErr = ErrAccessDenied
+	case ParentIsObject:
+		apiErr = ErrParentIsObject
 	case BucketNameInvalid:
 		apiErr = ErrInvalidBucketName
 	case BucketNotFound: