Ionic V4 error message

I have a login page with error messages

 async err => {
      const toast = await this.toastController.create({
        message: this.errorMessage = err.message,
        showCloseButton: true,
        position: 'middle',
        closeButtonText: 'Fermer'

everything is displayed but I do not know where the messages are because I want to change them.
maybe I can put my personal messages and call them instead?
I use firebase

thank you in advance