desktop.eclass: domenu, fix dying on non-existing files

desktop.eclass: domenu, fix dying on non-existing files

The weird logic in domenu had an explicit separate clause
for unsuccessful return on non-existing files.  This worked fine before
EAPI 4 since '|| die' was mandatory.  However, since 'doins' started
dying on its own, developers have assumed the same for 'domenu'
and stopped checking the exit status.  As a result, missing files
are now silently ignored.

Change the logic to explicitly die when the file does not exist.
To provide the best interoperability and avoid code duplication, just
let 'doins' die on its own.