[dynamoDB] Acquire Cancellation Reasons from a TransactionCanceledException

[dallbee]

Version Information
SDK: v0.23.1-0.20200714212036-c1ad81608c5c
Go: 1.14

Is your feature request related to a problem? Please describe.
It is difficult to disambiguate Transaction cancellation reasons. There's a similar issue in the AWS SDK v1, which appears to have been fixed in the v1.28 release but not mirrored to v2.

Describe the solution you'd like
I'd like to be able to unwrap the error from a transaction request to obtain the underlying reason for failure per-item in the transaction. It'd be great if there were a strongly typed approach to this.

I think the solution in the V1 SDK makes sense, so I'm really just hoping for a port of that functionality.

Describe alternatives you've considered
The JS SDK has a similar issue. The current suggestion there is to use string parsing, which is what we're currently doing in our project to handle this.

[jasdel]

The latest version (+ v0.25.0) of the v2 preview SDK now generates typed errors for all APIs. This includes the DynamoDB's Cancellation reasons in TransactionCanceledException error response.

[github-actions]

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[proxy-hatch]

any example for this?