net: sockets: Add extra debug print for invalid socket access

Print also thread name when printing socket access error.
This helps debugging what thread caused it.
Lowering the Error to Debug as we could be missing vtable
for example for eventfd descriptors, this is easily seen when using
socket service API.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>