Failed chunks should move offset backwards

If a chunk has failed it needs to move the offset backwards,
otherwise this would misalign the stream, or make it possible
to call sendNextChunk() right after resend which could be
seen as the chunkTimer attempted to be started while already running.

Change-Id: I6d44195ae9bcb23075ffea9c4f1d1ab1d0540c93
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
(cherry picked from commit 6d4d0642f80555c2b5c8d2ff87681b600a22b4a0)