Add oauth error_path

[comhar]

---

about: Handle OAuth Cancellations and Errors

Related Issue
#569

Proposed Changes
Update the OAuth redirect route so that it correctly handles oauth errors/cancellations.

In addition, this PR proposes an OAuth error_path parameter. The idea is that if the user hits cancel or some other error occurs, they will be redirected to error_path.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.

Additional Information
Any additional information, configuration or data that might be necessary to reproduce the issue.

[gitnotebooks]

Found 1 changed notebook. Review the changes at https://app.gitnotebooks.com/AnswerDotAI/fasthtml/pull/570

[johnowhitaker]

Looks good to me.