sched_ext: Improve error reporting during loading

When the BPF scheduler fails, ops.exit() allows rich error reporting through
scx_exit_info. Use scx.exit() path consistently for all failures which can
be caused by the BPF scheduler:

- scx_ops_error() is called after ops.init() and ops.cgroup_init() failure
  to record error information.

- ops.init_task() failure now uses scx_ops_error() instead of pr_err().

- The err_disable path updated to automatically trigger scx_ops_error() to
  cover cases that the error message hasn't already been generated and
  always return 0 indicating init success so that the error is reported
  through ops.exit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Vernet <void@manifault.com>
Cc: Daniel Hodges <hodges.daniel.scott@gmail.com>
Cc: Changwoo Min <multics69@gmail.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>