When another node is reported to be down, view updates queued
for it are cancelled, but some of them may already be initiated.
Right now, cancelling such a write resulted in an exception,
but on conceptual level it's not really an exception, since
this behaviour is expected.
Previous version of this patch was based on introducing a special
exception type that was later handled specially, but it's not clear
if it's a good direction. Instead, this patch simply makes this
path non-exceptional, as was originally done by Nadav in the first
version of the series that introduced handling unstarted write
cancellations. Additionally, a message containing the information
that a write is cancelled is logged with debug level.
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