lua: use internal error description for ClientError

box.error creates a ClientError unless custom type is provided.
There are two forms of ClientError creation:
* box.error(code, ...)
* box.error({code = code, ...})
The first form generates an error with reason that is constructed
from internal error description, and this is good.
The second one can use reason or message explicily provided in
argument table, but if none is provided generates an empty message.
That differs for the first behavior and hampers generation of
ClientError creation from lua.

This patch fixes that and uses internal error description for
ClientError if the message was not provided explicitly.

Closes #9876

NO_CHANGELOG=was not released yet
NO_DOC=bugfix