Make errorHandler take error instead of string

[kinbiko]

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Description

Make error handler take an error instead of a string. This is a much more powerful API, especially since Go 1.13, as nested errors can now be extracted. I didn't introduce any %w wrapped errors as I wasn't sure which versions of Go you support.

This is technically a breaking change, as it changes the accepted values given to ErrorHandler in the Options struct. I did notice that there hasn't been v1 release of this package yet (technically v0.0.0-whatever still as reported in go.mod) so I'm raising this PR in the hope that breaking changes are still accepted.

Changes error messages to start with a lower case character.

Added missing note about what the default value is for ErrorHandler

References

Fixes #52.

Testing

Change can be tested by using the package as described and passing in an Extractor in Options that always returns an error.
Tests pass on my local MBP dev machine.

• 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
  • I have not checked the docs on auth0.com/docs.
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

[d10i]

Would love for this to be merged.

[kinbiko]

I'm closing this PR since this library is not maintained, I'm no longer interested in using Auth0, and I don't feel like maintaining my fork.

The PR (and thus the diff) should still be available should the team choose to maintain the package in the future.