Scheduled Txs: Add event for failed executions

[joshuahannan]

Issue To Be Solved

Many implementations of scheduled transactions have organically emitted an event to emit when a scheduled transaction fails. It would be good to standardize this.

Suggest A Solution

An event + emitter method so block explorers could watch for interface-level events and add failure states.

// inside FlowTransactionScheduler

access(all) event HandlerExecutionFailed(
    id: UInt64,
    handlerType: String,
    address: Address?,
    uuid: UInt64,
    error: String
)

access(all) resource interface TransactionHandler {
    // ...
    // default implementation emits an event - can be overridden
    access(contract) fun emitExecutionFailed(
        id: UInt64,
        error: String
    ) {
        post {
            emit HandlerExecutionFailed(
                whileExecuting: whileExecuting,
                handlerType: self.getType().identifier,
                address: self.owner?.address,
                uuid: self.uuid,
                error: String
            )
        }
    }
}

// ---------

// inside consuming resource
access(all) resource COAHandler : TransactionHandler {
    access(FlowTransactionScheduler.Execute) fun executeTransaction(id: UInt64, data: AnyStruct?) {
        // some failure state encountered
        self.emitExecutionFailed(id: id, error: "CALL_FAILED")
        return
    }
}

The main concern would be id spoofing, but maybe there's something workable there to consider in the future.

This would be upgradeable