Cannot create Qt Designer Custom Widget project with CMake

Try creating a new Qt Designer Custom Widget using CMake as build system. Instead of creating the project, I run into an unclear "file not found" error:

I have cmake.exe in PATH and I can create a new Qt Widgets Application with it.

At the very least, the error message should tell the user what went wrong and how to solve that. If a Qt Designer Custom Widget cannot be created with CMake at all, the user should be informed about that earlier.

Found while extending Squish test tst_new_project_file_creation.