ki18n_wrap_ui: error when file doesn't exist

Summary:
Makes it clear why the current build is failing instead of the developer wasting
time investigating other causes. The message is styled similar to the CMake
error when it can't find a source file.

Test Plan:

Reviewers: Framework: Syntax Highlighting, Localization, Build System, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: cullmann, ngraham, kossebau, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26670