Make authN/authZ issues easier to diagnose

Many of the tests and examples can fail if the AWS SDK cannot find credentials, but it's difficult to root cause them since the error messages are often indecipherable or hidden. We should ensure make this failure mode easier to diagnose, preferably with consistent error codes/messages.

Start by reproducing the AWS SDK credential failure in the affected tests and examples, then trace how those failures are surfaced. Review the existing error output and identify where consistent error codes or messages could be exposed. Done means credential-related authN/authZ failures are easier to distinguish and diagnose.

Written by the indexing model from the issue text.