Throw an error when trying to remove the path for a dynamic node

[dumconstantin]

Given:

let db = DB({
  foo: 123
})
db.node('/bar', ['/foo'], x => x)

should throw an error (on /err/node) when attempting:

db.patch([{ op: 'remove', path: '/bar'}])

or any sub path starting at the dynamic node location.