Add task existence check before AWS CLI version check

[Anorlondo448]

Added a TASK existence check before checking the AWS CLI, because If the TASK specified in the second argument does not exist, the AWS CLI version upgrade message will be displayed.

$ bash check-ecs-exec.sh ecs-cluster-name aaaaaaaabbbbbbbbbbbbcccccccddddd
-------------------------------------------------------------
Prerequisites for check-ecs-exec.sh
-------------------------------------------------------------
  jq      | OK (~~~~)
  AWS CLI | OK (~~~~)
-------------------------------------------------------------
Prerequisites for the AWS CLI to use ECS Exec
-------------------------------------------------------------
Pre-flight check failed: ECS Exec requires the AWS CLI v1.19.28/v2.1.30 or later.
Please update the AWS CLI and try again?
  For v2: https://docs.aws.amazon.com/cli/latest/userguide/install-cliv2.html
  For v1: https://docs.aws.amazon.com/cli/latest/userguide/install-cliv1.html

[Anorlondo448]

@toricls
Thank you for your comment!
Fixed the message.

[toricls]

Thanks for the contribution @Anorlondo448 🙌