Add UpgradeErrorPath to 24-host

[damiannolan]

Summary

The upgrade error path is a public path that can signal an error of the upgrade to the counterparty for the given upgrade attempt. It does not store anything in the successful case, but it will store the ErrorReceipt in the case that a chain does not accept the proposed upgrade.

Add UpgradeErrorPath to 24-host with associated key func.

• UpgradeErrorPath
• UpgradeErrorKey

Ref: 24-host/keys.go

function errorPath(portIdentifier: Identifier, channelIdentifier: Identifier): Path {
    return "channelUpgrade/ports/{portIdentifier}/channels/{channelIdentifier}/upgradeError"
}

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For Admin Use

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[damiannolan]

Review spec for any updates to key paths