20318: Fix BadChecksum error not propagated through cache layer.

20318: Fix BadChecksum error not propagated through cache layer.

Previously, ReadAt would not return an error as long as enough bytes
were read, because the error might mean the read was truncated after
the point needed for the current read. This had the unintended
consequence that even a BadChecksum error returned on the final read
would not be propagated.

The updated version propagates errors even if the requested bytes were
retrieved, so the only case where a bad checksum error can be
undetected is when the read does not reach the end of the block *and*
the block has not been fully retrieved yet.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>