AWS::ApplicationSignals::ServiceLevelObjective - CREATE_FAILED Error Message Unclear

### Name of the resource

Other

### Resource name

AWS::ApplicationSignals::ServiceLevelObjective

### Description

CREATE_FAILED error message is unclear and needs more detail:
Resource handler returned message: "text" (RequestToken: 0ce758be-1cb0-953b-a040-c6d3b6f49511, HandlerErrorCode: InternalFailure)

This was thrown when a start time wasn't defined in the CFN template for an SLO exclusion window. In the documentation, it states that "StartTime" isn't required which is understandable when updating the CFN template. However, when creating the SLO for the first time through a CFN template, it should be noted in the error message that the StartTime needs to be included. I also confirmed that this was the case through creating a SLO exclusion window on the CloudWatch console.

### Other Details

Please slack me if you would like more detail.

Start by reviewing the AWS::ApplicationSignals::ServiceLevelObjective resource behavior for SLO exclusion windows created from a CloudFormation template, comparing it with the CloudWatch console behavior. Done means a creation failure identifies that StartTime is required when creating the exclusion window, while the existing update behavior remains clear.

Written by the indexing model from the issue text.