throw SwordError for unparseable Atom entry swordapp/JavaServer2.0#6

By throwing a SwordError when `org.apache.abdera.parser.ParseException` is thrown from `parser.parse(entryPart)` I'm able to handle the exception in my application code, as explained in this ticket:

Fix Abdera ArrayIndexOutOfBoundsException with non-existent atom-entry-study.xml in SWORD jar (upstream ideally) · Issue #893 · IQSS/dataverse - https://github.com/IQSS/dataverse/issues/893

In case it's not obvious from the commit message or the title of this pull request, the bug I'm trying to fix on the SWORD library side is this one:

feeding an empty Atom entry file to CollectionAPI.post results in org.apache.abdera.parser.ParseException: java.lang.ArrayIndexOutOfBoundsException · Issue #6 · swordapp/JavaServer2.0 - https://github.com/swordapp/JavaServer2.0/issues/6