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