box: raise ER_ITERATOR_POSITION on any kind of invalid position

Currently, if the position isn't compatible with the index, we raise
an error like "Invalid key part count ...". From this error it's
difficult to figure out whether it's for the given iterator position of
for the search key. Let's always raise ER_ITERATOR_POSITION in this
case. Later on we'll use stacked diag to add extra error info.

Needed for #8403
Needed for #8404

NO_DOC=bug fix
NO_CHANGELOG=unreleased