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/object-api-errors.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"fmt"
 	"io"
+	"path"
 )
 
 // Converts underlying storage error. Convenience function written to
@@ -47,6 +48,13 @@ func toObjectErr(err error, params ...string) error {
 				Object: params[1],
 			}
 		}
+	case errFileParentIsFile:
+		if len(params) >= 2 {
+			err = ParentIsObject{
+				Bucket: params[0],
+				Object: params[1],
+			}
+		}
 	case errIsNotRegular:
 		if len(params) >= 2 {
 			err = ObjectExistsAsDirectory{
@@ -182,6 +190,13 @@ func (e PrefixAccessDenied) Error() string {
 	return "Prefix access is denied: " + e.Bucket + "/" + e.Object
 }
 
+// ParentIsObject object access is denied.
+type ParentIsObject GenericError
+
+func (e ParentIsObject) Error() string {
+	return "Parent is object " + e.Bucket + "/" + path.Dir(e.Object)
+}
+
 // BucketExists bucket exists.
 type BucketExists GenericError