Improve IMDS error diagnosability

[chlowell]

ManagedIdentityCredential.GetToken() returns a credentialUnavailableError when IMDS returns a 400 response. credentialUnavailableError doesn't carry the raw response, preventing applications from programmatically inspecting it. The error type is correct--a 400 from IMDS mustn't be fatal when iterating a chain of credentials--but the response, if any, should be exposed to the application.