Unexpected error occurs in RSS block with negative Offset.

Steps:
1. Create a new Landing Page.
2. Add a RSS block.
3. Provide valid URL.
4. Submit.
5. Open settings form again.
6. Enter negative offset value.
7. Submit.
Actual: "An unexpected error has occurred" (critical exception attached).
Expected: Negative offset values are handled in a more user-friendly way.

Remarks: Please see attached.