[Reporting] Capture the screenshot of a page even when Reporting's render timeout happens

[tsullivan]

Describe the feature:
Today in Reporting, there are many safeguards in the pipeline of opening the Kibana URL, waiting for elements to render, and capturing the page with the browser driver screenshot method. These safeguards ensure that a PNG or PDF result doesn't show incorrect or missing information.

Increasingly, the safeguard methods have made it hard to troubleshoot problems that happen on the page. We should still have a timeout, and log warnings when timeouts happen, but we should still capture the page and deliver it to the user, along with the warnings in context.

The warnings could be shown as an overlay on the page PDF/PNG, and/or they could be shown in text along with the report information in the Reporting Management job listing table.

Describe a specific use case for the feature:

1. A user has a Kibana server with no outbound network access
2. They have a dashboard with a Maps visualization which requires network access to load
3. In the screenshot capturing pipeline, the Maps can't load assets from the Internet. The "render complete" event never fires for the Maps visualization.
4. The screenshot capture pipeline hits the timeout, but instead of failing the job, it captures what is on the page and overlays warnings about something that didn't complete fully.
5. The user can troubleshoot their setup based on what they see in the incomplete PDF/PNG.

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)