…erAsyncReader

The last_read_ struct records information about the last call of
AsyncRead() which is a DownloadResumerAsyncReader's function. It
makes more sense to record this information inside that class
then. The strange `friend` declaration allowing a
DownloadResumerAsyncReader instance contained inside
DownloadResumerClient to access DownloadResumerClient's private
members can now be dropped and the hierarchy makes more sense.

Ticket: MEN-9954
Changelog: None
Signed-off-by: Vratislav Podzimek <vratislav.podzimek+auto-signed@northern.tech>

…stream

When there are network issues during Artifact download and the
HTTP download resumer exhausts its retry backoff, the only way to
deliver the error to the state machine is through the HTTP **body
reader's** handler because the **body handler** passed to the
DownloadResumerClient is only called when all body data is
fetched which normally happens in a completely different state
than the one that starts the download and thus the
UpdateDownloadState doesn't handle any errors in it.

Add a Fail() function to the body reader and use it to make sure
it calls its handler with the respective error.

Also add a comment about how the DownloadResumerClient is used.

Ticket: MEN-9954
Changelog: Fixed mender-update hanging indefinitely (never reporting a
deployment failure and never returning to the poll loop) when a network
outage during artifact download lasted long enough for the download
resumer to give up on resuming.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Vratislav Podzimek <vratislav.podzimek+auto-signed@northern.tech>

…:Read()

The handler passed to the asynchronous reader can be called
multiple times after adding a call to `AsyncRead()` to the event
loop, not only once when there is data available, but also if the
asynchronous reader encounters an error. Since the event loop is
only run recursively **once** per ReaderFromAsyncReader::Read()
call, we need to make sure it's also stopped only once. Otherwise
it may stop the event loop for good.

Ticket: MEN-9954
Changelog: None
Signed-off-by: Vratislav Podzimek <vratislav.podzimek+auto-signed@northern.tech>