Export API: warn when omitting works from full publisher-metadata record

[rhigman]

If a user requests the metadata record for a work in a certain format and the work is missing a required field for that format (e.g. PDF publication URL for ONIX, Landing Page or Publication Date for KBART), an error message will be returned.

However, if a user requests the metadata record for all works by a given publisher, and one or more of these works are missing a required field, a record will be successfully returned with these works silently omitted.

Ideally, when we return a full publisher-metadata record, we should also return a list of which works were omitted, and what data was missing that resulted in them being omitted, so that the user is not unknowingly submitting incomplete lists of works to other platforms.

Relatedly, if every work by a given publisher is missing a required field, an empty publisher-metadata record will be returned, but with no error message or other warning.

[rhigman]

#325 introduces another route by which a silently empty publisher-metadata record may be returned: if all of a publisher's records are Chapters.

[ja573]

There might be situations in which a publisher may want to get the output as it is, but it'd be better if the default behaviour was to output the error if any book has errors. Maybe we could have a flag (e.g. permissive, strict, or dry-run) to toggle its permissiveness

[rhigman]

There are also cases where hitting an error within a work breaks the ONIX formatting of the full publisher record, if a partial record has already been written to the stream when the error is encountered (e.g. missing USD price for Overdrive, which is not checked before the record writing begins, unlike e.g. missing publication date).