Bad error ER_VIEW_INVALID returned upon attempt to insert into mysql.user

MDEV-19650 introduced a new definer for mysql.user view, with just enough permissions to perform valid operations on the view.

A side effect of it is that an attempt to insert into the view (which is an invalid operation) started producing a bad error code. Instead of the former accurate ER_NON_INSERTABLE_TABLE ("The target table user of the INSERT is not insertable-into") it now returns the less user-friendly ER_VIEW_INVALID ("View 'mysql.user' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them").