fix: Add cause to Cognito and STS error messages

[qhanam]

Cognito and STS error messages do not currently contain the underlying error.

This change adds the underlying error to the thrown error message.

This change also addresses three other issues in the authentication code:

1. Fix an asynchrony issue where errors weren't being handled by the catch block.
2. Refactor async functions to use async style instead of promise style.
3. Fix linter warnings by adding explicit types to responses.

Resolves #227 and #233.

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