fix(storage): Don't return empty credential while loading failed

[Xuanwo]

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Summary

OpenDAL currently returns an empty credential when COS loading fails—for instance, if there's an error with COS IAM. As a result, this triggers a Permission Denied error on the Databend side.

This PR modifies OpenDAL's behavior to return a retryable error instead, allowing OpenDAL to internally retry the request.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - OpenDAL related changes.

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):