Shell console: fix bug in shell output stream

The command is finished and the partial result may also have the command
exit code. When the partial stream does have the exit code, we were
writing *only* the contents of the exit code, but we need to write
everything *until* the exit code, keeping the contents intact.

LETTUCE-755
Change-Id: I5e7af1125270c394f42fe7661cbefdd98c6a6052
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit 22b9eab3)