Displaying more error messages on the bot discord command and frontend

[TGuillouet]

We should display errors when:

• The user is using the onboard command
  • If there is no linked promotion to the entered email
• The user is on the page that will change it's role
  • An error occurred while updating the user on the back-end (In this case, using the error messages returned by the back-end)
  • An error occurred while updating the user's discord name and roles (use the error message from discord-js ?)