Fix #88 - Pass errors to next in express

[zxlin]

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Description

This PR specifically addresses #88

Errors produced in the auth middleware will now be passed into next in the express middleware chain.

This is likely considered a breaking change as behavior that was once handled by the lib will now be the responsibility of the end user. However, this adds customizability to the user of the lib to allow custom error handling and such.

References

See issue #88

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

[zxlin]

hi @ziluvatar & @luuuis, who would be the best to review this change?

Thanks!