Clean up state of libxml on failed import.

Clean up state of libxml on failed import.

Make sure we always call XMLReader::close() to clean up libxml's internal state,
even if import fails with an exception. Otherwise, any subsequent attempt at importing
(or otherwise using an XMLReader) will fail with:

  XMLReader::open(): Unable to open source data

This is particularly annoying for unit tests which should be allowed to fail
without dragging down subsequent tests. Even more importantly, they may
explicitly be testing a failure case, which should not cause subsequent tests
to fail.

NOTE: Wikibase patch Id035ecebebb67 is blocked on this,
please re-check once this is merged.

Change-Id: I31c014df39aa11c11ded70050ef12a8e2c5fefc5