refactor onError relabeling of DestFile as '.FAILED'

This helps ensure three things:
- each error is reported via ReportMirrorFailure
- if DestFile doesn't exist, do not attempt rename
- renames happen for every error

The last one wasn't the case for Size mismatches, which isn't nice, but
not a exploitable problem per-se as the file isn't picked up and remains
in partial/ where the following download-try will at most take it for a
partial request which fails the hashsum verification later on

Git-Dch: Ignore