C#/Unity SDK callback for schema mismatch

[jdetter]

Steve from the BitCraft team raised this PR which calls the OnUnhandledReducerError when a schema mismatch caused a transaction update error in the SDK: clockworklabs/com.clockworklabs.spacetimedbsdk#356

IMO I think we should have a specific callback for this general scenario, possibly something like OnTransactionError or something like that where we can pass these sorts of errors back to the client.

TODO

• Determine the signature for this new callback. Consider also that we may in the future need to implement this callback in all SDKs but for now we are just focusing on the C#/Unity SDK.
• Implement the callback

Nice to have

• Testsuite test which demonstrates that the new callback works