[python][packagemetadata] Do not raise exception when non-standard dune.module keys are present

With this patch, we still do not add them to the metadata but write a log entry instead of raising an exception

I ran into this with an older setup and opm-common. opm-common has a non-standard key Label: in dune.module and the script then exits here, although, in my opinion, this shouldn't cause a fatal error.