minor mycore-oai changes

Description

-MCROAISolrSearcher#getEarliestDatestamp is not working with solr 7.0
-introduce MCR.OAIDataProvider.FailOnErrorRecords to retrieve better error messages if one record is invalid or to skip them silently

Environment

None

Assignee

Matthias Eichner

Reporter

Matthias Eichner

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External issue ID

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Components

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Priority

Medium
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