Change the impossible-diff message.
Change the impossible-diff message.

Fixes bbc/microbit-extras #1808.

SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,jle33/TouchDevelop,jle33/TouchDevelop,jle33/TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,jle33/TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,jle33/TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop,SoftwareEngineeringToolDemos/FSE-2012-TouchDevelop