Add error handling and reporting subsections

[mattgarrish]

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I've made the error reporting section informative since we're not recommending or requiring it.

Fixes #2454

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[mattgarrish]

I've done some rewriting of the section to focus it more on debugging. I brought in web dev tools and mention providing access to issues from, for example, the HTML, CSS and JS rendering, but I'm not sure if we want to get any deeper into it than that. I also mention epubcheck as a source of validation info.

If you have any more ideas on what to say here, though, feel free to offer them up!

[mattgarrish]

I also added a subsection for all the text that precedes the error sections (called "requirements" for lack of a better name). It looked strange in the table to have a conformance section with only two error reporting subsections (the stray paragraph problem that means you can't cite those requirements without also referring to all of section 2).

[iherman]

I also added a subsection for all the text that precedes the error sections (called "requirements" for lack of a better name). It looked strange in the table to have a conformance section with only two error reporting subsections (the stray paragraph problem that means you can't cite those requirements without also referring to all of section 2).

I like it as is. As far as I am concerned, it is a go!

Thanks @mattgarrish

[iherman]

@mattgarrish @wareid I think this can be merged now.