chore(storage): Improve error messaging for MP upload missing ETag

[cshfang]

Description of changes

This PR improves the error messaging around missing ETag parameter when completing multipart uploads. When using non-Amplify configured S3 buckets, it is possible to miss configuring the ExposeHeaders element of the bucket CORS configuration. When hitting this error, the messaging is too vague: Invalid parameter for CompleteMultipartUpload API and the input is logged as [Object object]. This PR adds more context specifically around the missing ETag parameter and stringifies the input for better debugging clarity in the future.

Issue #, if available

#13409

Description of how you validated changes

Reproduced the error by using Amplify to upload to a S3 bucket without ETag as exposed header in CORS configuration. Validated that:

1. The more specific error message now shows up when ETag is missing
2. The input object is now stringified

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[AllanZhengYP]

Looks good to me.