error: export error_unref() function

C struct error objects can be created directly only in C.
C-side increments their reference counter when pushes to the Lua
stack.

It is not going to be so convenient soon. error_unpack() function
will be used in netbox to decode error object via Lua FFI.

Such error object will have 0 refs and no Lua GC callback
established. Because it won't be pushed on Lua stack natually,
from Lua C. To make such errors alive their reference counter
will be incremented and error_unref() will be set as GC callback.

Follow up for #4398