REST: set error message extenesion for bad-request response log

REST: set error message extenesion for bad-request response log

We send it already to the user via the response body, but the
log_response does not has, nor wants to have FWIW, access to the
async body stream, so pass it through the ErrorMessageExtension
mechanism like we do else where.

Note that this is not only useful for PBS API proxy/daemon but also
the REST server of the file-restore daemon running inside the restore
VM, and it really is *very* helpful to debug things there..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>