discover/boot: Safely cleanup after failed load

If a call to load_url_async() fails immediately, boot() will free the
boot task and return. If other jobs started by load_url_async()
are still running they will attempt to free their task struct in
load_url_process_exit(), however the original boot task is the parent
context of this process task, resulting in a double-free.

Instead call cleanup_cancellations if an error immediately occurs to
cancel any pending load operations safely before freeing the boot task.

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>