Website UI: Explain the Logs modal

[adamziel]

Motivation for the change, related issues

@juanmaguitar asked:

What's the purpose of the "View logs" option available in https://playground.wordpress.net/
The modal that appears when clicking that option doesn't seem to do nothing.

It's not the first time when this question comes up, so I added a placeholder text whenever there are no logs to display:

Testing Instructions (or ideally a Blueprint)

1. Go to the logs modal after Playground is loaded
2. Confirm the placeholder text shows up
3. Trigger an error in devtools: setTimeout(() => {throw new Error('test');});
4. Confirm the error showed up in the modal
5. Search the modal with a query that doesn't match any log line
6. Confirm you got a "No matching logs found." message