fix: Differentiate error messages depending on required credential type

[mattkram]

Summary

Currently, our request callbacks that handle 401/403 server errors currently do not differentiate between repo tokens and API keys. This is a problem because the instructions are slightly different for how the user should resolve their issue.

• If they need to get a new API key, they have to do anaconda login
• If they have to get a new repo token, they currently have to do anaconda token install

This will remain this way until we unify on API keys for all repository access.

This PR refactors in addition to #196 such that the credential type required for the channel is available inside the __call__() method. Then, we can construct a callback hook function closure containing the value, which is retrieved if the callback is invoked.

There might be more opportunity to cache or otherwise ensure performance of this hook function.

I've extended the tests to test the error messages for the following matrix of cases:

• Credential was or was not sent
• Channel requires repo token or API key
• 401 or 403 response