Remove unwraps via '?' with anyhow crate for example-oauth

[rsdlt]

Refs: #2039

Motivation

Substitutes unwrap calls with proper error handling via the anyhow crate for example-oauth.

Solution

Substitute unwraps with ? via anyhow. Using if let or match is very verbose and makes the example lose focus. Using ? keeps the example readable, focused and shows better practices.

Validated solution by compiling and testing via CLIENT_ID=REPLACE_ME CLIENT_SECRET=REPLACE_ME cargo run -p example-oauth

[davidpdrsn]

Thanks for working on this!

In general I think we should also use .context("...")? to make it clear where the error is coming from. Otherwise it can be hard to debug later.

[davidpdrsn]

Thanks!