box: fix box.iproto.override crash if used before box.cfg

The function can't be called on an unconfigured instance because it
needs IPROTO threads up and running. Let's raise an error to avoid
a crash.

Since we have two other places where we need to raise the same error
(box.session.su and box.__index), let's introduce the new code
ER_UNCONFIGURED for this error.

Closes #8975

NO_DOC=bug fix

(cherry picked from commit 4fd2686e)