add error hex to hex tool page

Author: microbit-mark

software/universal-hex-tool.md

@@ -14,4 +14,8 @@ The Universal Hex Creator lets you combine a hex file created for a micro:bit V1
 
 DAPLink will process the Universal Hex and only write data to the relevant board revision; V1 or V2. More information is available on our [Hex format](../hex-format), the [Javascript/Typescript libary](https://github.com/microbit-foundation/microbit-universal-hex) on which the Universal Hex Creator tool is based and the associated [Universal Hex specification](https://github.com/microbit-foundation/spec-universal-hex).
 
+To support [cross-device compatibility](../hex-format/#cross-device-compatibility), we have created a [standalone error hex](/docs/software/assets/stand-alone-error-v1.hex) that can be combined with a V2 only hex in this tool to produce a Hex that will work on a V2 board, but error if used on a V1.
+
+[Download standalone error hex](/docs/software/assets/stand-alone-error-v1.hex){: .btn.sm-btn download}
+
 {% include uhex-tool.html %}