[Reporting] A report for a deleted Canvas workpad fails for the wrong reason

[tsullivan]

Kibana version: master / 7.4.0

Elasticsearch version: n/a

Server OS version: n/a

Browser version: n/a

Browser OS version: n/a

Original install method (e.g. download page, yum, from source, etc.): source

Describe the bug:
Reporting detects problems on the Kibana page by looking for euiToastHeader elements, which are unexpected when Kibana is rendering a visualization for export. This helps the situation when a POST URL has been saved for a reporting job, but the underlying visualization or saved object in Kibana has been deleted. However, this is not working in Canvas, because Canvas doesn't always use euiToastHeader

Steps to reproduce:

1. Generate a POST URL for a Canvas workpad PDF report
2. Delete the workpad
3. Run the POST URL

Expected behavior:
Reporting should fail with a Saved object is missing message. Instead, it fails with a timeout error.

The only helpful message here is a debug level message:
server log [10:56:09.194] [debug][browser][execute][jzvk8xcl0n2rcbae0cfn0lcf][printable_pdf][reporting] Failed to load resource: the server responded with a status of 404 (Not Found)

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-canvas

[timductive]

Related to #43715

[tsullivan]

This is a low priority issue for Reporting. The 404 puts an error on the browser javascript console, which the Reporting headless browser picks up and logs as an error.

So the failure reason will still be timeout, but there is more information in the logs by default now.