Index resource even with partial parsing failures

There are collections which have an incorrect combination of access
status and locator - namely, no locator is present, but the access
status is either public or private. However, there are also referenced
thumbnails, which were consequently ignored until now. This however
leads to the problem that the thumbnail binaries written by the Media converter to the media directory are somewhat orphaned, which is not exactly optimal. Therefore this patch allows for thumbnail or audio snippet metadata entry written to the media server index even when parsing failures in the digital object had arisen.
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