set failreasons similar to connect.cc based on curl errors

Detecting network errors has some benefits in the acquire system as if
we can't connect to a host trying it for a million files is pointless.
http and co which use connect.cc deal with this, but https which uses
curl had connection failures as "normal" errors which could potentially
be worked around (like trying Release instead of the failed InRelease).

Git-Dch: Ignore

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