Fix: report error if consumer can't be spawned

Fix: report error if consumer can't be spawned

On error of the UST consumer execl(), we now exit the forked process
like the kernel does.

Furthermore, fix an error handling value when timing out when waiting
for the consumer to bootstrap thus returning an error to the client and
not continuing with the command thinking that the consumer was actually
spawned.

So, the expected behavior is to wait for the pthread condition and if a
timeout is seen, the client is informed thus making the client command
hang for the default timeout value of 30 seconds.

Fixes #702

Signed-off-by: David Goulet <dgoulet@efficios.com>