client: Improve error diagnostics.

client: Improve error diagnostics.

If an error occurs in the post_select() method of the client task,
the scheduler removes this task from the task list and then returns
zero to indicate that the task list has become empty.

Currently, client.c looks only at the return value of schedule() and
therefore returns success in this case.  This patch makes client.c
additionally check the task error state of the client task and
prints an error message if it was negative and different from an
EOF condition.