Builds: Add notification if PDF build command fails (!= 0)

[humitos]

Some time ago we made the build to fail if the PDF command was different than 0: #10113. However, we had to revert this because it affected a lot of users and the instructions about how to solve the problem was not clear. See #10158

So, we want to discuss if we want to deprecate this functionality, we need to do more work to move it forward. If not, we can keep the status quo.

Some potential work here:

• Decide if we are just skipping on this change
• Notify users of the change so they can address issues

I think we should probably use the our new and shiny "deprecation path" that involves sending weekly emails to the affected projects/users, plus writing a blog post with exact dates, etc.

[humitos]

We talked about this in a meeting and some ideas came up:

• create a fail-on-warning config key specifically for PDF
• keep the current behavior and show a build notification explaining there was a LaTeX error, but a PDF was created anyways and Read the Docs didn't fail the build

Since creating a new config key is not a trivial amount of work, I understand we will start with a notification first and see how it goes.

[humitos]

keep the current behavior and show a build notification explaining there was a LaTeX error, but a PDF was created anyways and Read the Docs didn't fail the build

This is related with the new build notification system: #10598

[ericholscher]

As we build build.jobs.build.pdf we will plan to respect the error codes, the main thing here is backwards compat for LaTeX which has wonky error codes.

We could also add a new notification that warns users about this.