Doing the tidy-up after the last check has the problem that it gets
skipped if any check fails, as the premature return skips the tidy-up.
In the process, added the missing tidy-up to prematureExec().
Repackage the code for one tidy-up as a named lambda so that the
duplicate can share it with the scope guard. Made some existing scope
guards const, while I was about it, so that the one that isn't -
because it gets dismiss()ed - stand out.
Change-Id: I96d6834d5d7675f15018169a7093b0211db6f8a9
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
9030ffdffb