Make ErrorDetails into a pointer

[rynowak]

Description

Since ErrorDetails implements error, it should really be used with a pointer everywhere.

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
• If applicable, design document has been reviewed and approved by Radius maintainers/approvers.
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.