revise: changes anyhow out for miette

[claymcleod]

Converts the errors used internally from anyhow to miette.

This doesn't affect much except the examples, but I felt it was most consistent to use miette since we've switched to using it in our other projects.

Notably, reqwest_middleware still uses anyhow, so it's kept around as a dependency.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have added at least one relevant code reviewer to the PR.
• Your code builds clean without any errors or warnings.
• You have added tests (when appropriate).
• You have updated the README or other documentation to account for these
  changes (when appropriate).
• You have added an entry to the relevant CHANGELOG.md (see
  "keep a changelog" for more information).
• Your commit messages follow the conventional commit style.