migration: push Error **errp into vmstate_subsection_load()

migration: push Error **errp into vmstate_subsection_load()

This is an incremental step in converting vmstate loading
code to report error via Error objects instead of directly
printing it to console/monitor.
It is ensured that vmstate_subsection_load() must report an error
in errp, in case of failure.

The errors are temporarily reported using error_report_err().
This is removed in the subsequent patches in this series,
when we are actually able to propagate the error to the calling
function using errp.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Arun Menon <armenon@redhat.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20250918-propagate_tpm_error-v14-1-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>