Delay.cancelExecution should revert on a non-existent script id
`cancelExecution` makes no existence check on the passed-in script ID, clearing its storage slot and emitting an event:\\n```\\n/\\*\\*\\n\\* @notice Cancel script execution with ID `\\_delayedScriptId`\\n\\* @param \\_delayedScriptId The ID of the script execution to cancel\\n\\*/\\nfunction cancelExecution(uint256 \\_delayedScriptId) external auth(CANCEL\\_EXECUTION\\_ROLE) {\\n    delete delayedScripts[\\_delayedScriptId];\\n\\n    emit ExecutionCancelled(\\_delayedScriptId);\\n}\\n```\\n
Add a check that the passed-in script exists.
```\\n/\\*\\*\\n\\* @notice Cancel script execution with ID `\\_delayedScriptId`\\n\\* @param \\_delayedScriptId The ID of the script execution to cancel\\n\\*/\\nfunction cancelExecution(uint256 \\_delayedScriptId) external auth(CANCEL\\_EXECUTION\\_ROLE) {\\n    delete delayedScripts[\\_delayedScriptId];\\n\\n    emit ExecutionCancelled(\\_delayedScriptId);\\n}\\n```\\n