Added error handling to useUser [SDK-2236]

[Widcket]

Description

This PR adds an error property to the return object of the useUser hook, that will contain an Error instance when the fetch() or response.json() calls throw.
The tests, documentation and example apps were updated accordingly.

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

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