Misleading error message when root volume is full

I was trying to use the SSM agent Run Command functionality and would get the following message for any kind of document execution:

> Plugin with name aws:runShellScript not found. Step name: aws:runShellScript

It wasn't specific to `aws:runShellScript`. In the end, the root cause was that the root EBS volume had no free block. It's not likely a common issue, but it'd be great to have an error message that reflected the actual cause of the issue.

Start with the SSM agent Run Command path for document execution, especially failures involving aws:runShellScript, and reproduce the behavior with the root EBS volume out of free blocks. Done means the reported failure identifies root-volume exhaustion instead of incorrectly saying the plugin was not found.

Written by the indexing model from the issue text.