fix: add more error handling to oaipmhimpl

...c/main/java/info/textgrid/middleware/OaipmhConstants.java

...re/src/main/java/info/textgrid/middleware/OaipmhImpl.java

.../java/info/textgrid/middleware/RecordListDelivererDC.java

.../main/java/info/textgrid/middleware/SetListDeliverer.java

...extgrid/middleware/test/online/OaipmhUtilitiesOnline.java

...dleware/test/online/tg/TGOaipmhResumptionTokenThread.java

...iddleware/test/online/tg/TestTGListIdentifiersOnline.java

...id/middleware/test/online/tg/TestTGListRecordsOnline.java