bsdinstall/distfetch: fix main bar percentage with errors

UI fix not related to the real fetching process, use 'nfiles'
(instead of 'total files size') to compute main bar percentage
if an error occurs:

• fix: main bar greater than 100%, if an error occurs before fetching
• fix: main bar less than 100%, if an error occurs during fetching
• add: last mixedgauge, at least one dialog if a total failure occurs

PR: 164094, 169748
Approved by: bapt (mentor)
Review: https://reviews.freebsd.org/D33978