net.box: fix box.error() usage

box.error() expects first argument to be numeric or table; passing
string to it is wrong. Accidentally in net.box module two places where
string is passed to box.error() as first argument: in :unprepare() and
:connect() methods. Let's fix them and pass error code ER_ILLEGAL_PARAMS.

NO_DOC=<bugfix>
NO_CHANGELOG=<Minor>