[easy-ssh] exit when targeted instance unreachable

[KeitaW]

This PR proposes easy-ssh.sh script to exit with error when targeted instance is unreachable.

Before:

❯ ./easy-ssh.sh -c controller-machine --region us-east-2 "ml-cluster-p5en"

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==== 🚀 HyperPod Cluster Easy SSH Script! 🚀 ====


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An error occurred (ResourceNotFound) when calling the DescribeCluster operation: Cluster with name ml-cluster-p5en not found

An error occurred (ResourceNotFound) when calling the ListClusterNodes operation: Cluster with name ml-cluster-p5en not found
Cluster id: 
Instance id: 
Node Group: controller-machine
Would you like to add ml-cluster-p5en to  ~/.ssh/config (yes/no)?
> 

After:

 ./easy-ssh.sh -c controller-machine --region us-east-2 "ml-cluster-p5en"

=================================================

==== 🚀 HyperPod Cluster Easy SSH Script! 🚀 ====


=================================================

An error occurred (ResourceNotFound) when calling the DescribeCluster operation: Cluster with name ml-cluster-p5en not found

An error occurred (ResourceNotFound) when calling the ListClusterNodes operation: Cluster with name ml-cluster-p5en not found
Error: Cluster or instance not found for the specified cluster name (ml-cluster-p5en). Exiting.

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