Unexpected return values from DataCiteEvent::datacite_doi

[jesusbagpuss]

[NB the following was discovered by code inspection, and probably doesn't make anything specifically 'broken' - but reporting of failures would be improved by resolving this]

The following lines return unexpected values:

• DataCiteDoi/lib/plugins/EPrints/Plugin/Event/DataCiteEvent.pm

  Line 66 in 570925d

  return $thisdoi if($thisdoi !~ /^$prefix/);

• DataCiteDoi/lib/plugins/EPrints/Plugin/Event/DataCiteEvent.pm

  Line 70 in 570925d

  return $xml if( $xml =~ /^\d+$/ ); # just a number? coin_doi has passed back an error code pass it on...

The return value from an EventQueue action should be an HTTP code:
https://github.com/eprints/eprints/blob/3.3/perl_lib/EPrints/DataObj/EventQueue.pm#L138-L151

I think that if an event action dies then the error gets logged in the EventQueue object's description field.

Resolution would be to:

• set an expected return code
• set the EventQueue 'desciption' field to include useful information (visible in the EPrints interface when viewing tasks)
• use $repo->log to make something appear in indexer.log file