fix: don't transform a ProfileDidNotContainCredentials error if base_provider...

fix: don't transform a ProfileDidNotContainCredentials error if base_provider is running for first provider in chain (#873)

* fix: don't transform a ProfileDidNotContainCredentials if base_provider is running for first provider in chain
add: more comments to aws_config::profile::credentials::repr functions

* add: Test ensuring provider chain hits IMDS even if some config data exists

* update: test to expect success response

* add: clarifying comment about chain vs base providers