Return different errors based on whether the API key was provided

[mmarchetti]

Intent

This PR causes the test-credentials API to return a different error if the request fails, based on whether an API key was provided or only a server URL.

Fixes #1875

Type of Change

• • [x] Bug Fix
• • [ ] New Feature
• • [ ] Breaking Change
• • [ ] Documentation
• • [ ] Refactor
• • [ ] Tooling

Automated Tests

Added tests to cover all 4 combinations of

• API key provided or not
• Server responds with 401 or 404

Directions for Reviewers

it's easiest to test this with the new UI in https://github.com/posit-dev/publisher/pull/1882.