Patch from Tim Kosse: check back->exitcode() in both loops on

Patch from Tim Kosse: check back->exitcode() in both loops on
ssh_sftp_loop_iteration(), not just one. Fixes exiting on a negative
response to the host key confirmation prompt on Windows (because
winsftp.c doesn't have the equivalent of uxsftp.c's no_fds_ok); on
Unix it worked already but gave a suboptimal error message, which is
fixed too by this patch.

[originally from svn r8110]