`credentials.Chain` doesn't return any error if it fails to resolve

[acolombier]

When using credentials.Chain (ref), if all the provider returns an error, an anonymous value is returned instead of an error.

Looking at the tests (here and here), it feels like this was intended to returns an error, in case of all provider failing (or none were set).

Is this correct? Would you accept a fix for RetrieveWithCredContext to return an error if all providers have returned an error?