feat: Errors now wrap underlying errors

[mitar]

Related issue

Fixes #458.

Proposed changes

All errors wrap now underlying errors, when there is one available. This is not directly exposed through messages, but if you obtain such error in your code you have more information to inspect, log, and act upon.

I also changed around the codebase to remove hard-coded assumptions about the depth of a cause when inspecting errors and use errors.Is and errors.As instead.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)

[aeneasr]

Awesome, thank you for your contribution!