Improve error diagnostics when input or output files are missing
Summary:
qFatal results in [c]make just showing "core dumped", exiting with a
non-zero error code shows the actual error message instead.
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: dvratil, kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D20252