Fix bug: default credential provider hides sub credential provider's fetching
credentials error.

The default credential provider will try to rescue sub provider's
MissingCredentialsError for looking up the right credential provider.
However, the way using 'rescue' keyword is not correct, and it
actually rescued all the StandardError raised from sub provider's
credentials method.
Merge pull request #147 from ThoughtWorksStudios/credentials_provider_bug

Fix bug: default credential provider hides sub credential provider's fetching credentials error