Bug 1875248 - Check for network error preventing ExternalHelperAppService...

Bug 1875248 - Check for network error preventing ExternalHelperAppService before DONT_RETARGET, r=smaug

This reverts the change from 30cde47f9364e5c7da78fd08fa8ab21737d22399,
and instead re-orders the NS_ERROR_FILE_NOT_FOUND check before
DONT_RETARGET.

Testing suggests that a-download-click-404.html behaviour isn't
impacted, and this improves the handling of this edge-case when doing
process switching.

Differential Revision: https://phabricator.services.mozilla.com/D202007