External image not found: better error message

[pgundlach]

<Layout xmlns="urn:speedata.de:2009/publisher/en"
    xmlns:sd="urn:speedata:2009/publisher/functions/en">

    <Record element="data">
        <PlaceObject>
            <Image file="https://doesnotexist.doesnotexist"></Image>
        </PlaceObject>
    </Record>
</Layout>

returns

Error: [page 1] ...rs/patrick/work/software/publisher/src/lua/publisher.lua:6674: attempt to index a nil value (local 'f')
stack traceback:
        ...ick/work/software/publisher/src/lua/publisher/spinit.lua:54: in function 'call'
        ...ick/work/software/publisher/src/lua/publisher/spinit.lua:389: in function 'main_loop'
        ...ick/work/software/publisher/src/lua/publisher/spinit.lua:398: in main chunk
        [C]: in function 'require'
        [\directlua]:1: in main chunk
Stop processing data

The software should handle this a better.