[REQUIRED] Describe your environment

• Operating System version: _____
• Browser version: _____
• Firebase SDK version: _____
• Firebase Product: _____ (auth, database, storage, etc)

[REQUIRED] Describe the problem

v8:

v9:

The change of error code format (it attaches "functions/" at the beginning since v9) is not mentioned in the v8 v9 migration guide. It should be well announced in the documentation.
Otherwise, if a user relies on the error code string in their app, the upgrade will cause a crush (like me 😢 ).

Steps to reproduce:

Relevant Code:

// TODO(you): code here to reproduce the problem