JavaScript TypeError, prints object as '#

Probably a naive question - if this snippet is executed (I'm running with Node.js v12.16.3, same result in Chrome apart from the error type):

const obj = {}

Object.defineProperty(obj, 'a', {
  writable: false,
  value: 13
})

obj.a = 14

It will, obviously, throw an error, but the message string of that error has strange representation of an object:

TypeError: Cannot assign to read only property 'a' of object '#<Object>'

The question is - why it's stringified as #<Object>?

Defining a toString method on the object will render it as if it were called with Object.prototype.toString.call(obj):

const obj = {
  toString () {
    return 'stringified obj'
  }
}

Object.defineProperty(obj, 'a', {
  writable: false,
  value: 13
})

console.log(`${obj}`)

obj.a = 14

TypeError: Cannot assign to read only property 'a' of object '[object Object]'

While console.log outputs correctly. How to make that TypeError output the correct string representation? What does it use internally?

from JavaScript TypeError, prints object as '#