MidlRT has a bad error message when Platform XML can't be parsed

[Mark Schofield]

When attempting to read a specific SDK's 'Platform.xml' file, the following error is logged when the file isn't found:

CMake Error at C:/Users/markt/source/repos/Monstrosity/WindowsCMake/WindowsCMake/MidlRT.cmake:45 (string):
  string sub-command REPLACE requires at least four arguments.

WindowsCMake/MidlRT.cmake The PowerShell should've failed, but error handling isn't present so things fail cryptically later.