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.

The fix corrects the syntax of using rescue with a begin block to ignore MissingCredentialsError