Commit bd7aab73 added
support for pre-fetching sources from distfiles.

In the event that the distfiles source archive is stale,
e.g. if autobuilder is run before the 'healthchecks'
cron job or a downstream mirror is synced, new sources
were downloaded as 404 HTML files.

When the package itself was built, it failed because a
HTML masquerading as a .tar.gz cannot be extracted.

This commit silently ignores 404'd distfiles hits and
relies on 'abuild fetch' to download the actual source.