While two pdf.js versions broke each other, every PDF preview rendered
after a local text read failed and was recorded as raster-failed, which
the thumbnail backfill treats as final. Migration 0478 deletes those
failure rows, so the boot-time thumbnail backfill finds each document
again and renders it once. A stored failure cannot tell the clash from a
page that really renders blank; such a page fails once more and settles
with one attempt. Only raster-failed is cleared: the clash surfaced as a
failed rasterization, which the thumbnail path maps to that reason alone.
Refs #1277
