SP3 center that did not contribute to a merge.
reason is "no_candidate" (the catalog names no candidate for the center
and date), "catalog_unavailable" (the center has no cataloged naming
convention for the date), "offline_cache_miss", "product_not_published",
"checksum_mismatch", "http_status" or "unclassified"; the last five
name the failed candidate in filename, pattern and url, with its HTTP
status in http_status where it has one. An "unclassified" failure, one
no other case describes, carries the inspected reason in detail, which is
nil otherwise.