Fail early with clear message if book header structure is not the expected one

[cderv]

This follows an investigation from this question
https://community.rstudio.com/t/bookdown-debugging-fixing-incomprehensible-error/87014/6

• The book start with a header h2. By default, split_by = "chapter" so a h1 is expected - this fails with a cryptic error (see community question)
• If the book start with a header h2 but split_by: section is set, the render stop early because of what I believe to be a wrong check
  

  bookdown/R/html.R

  Lines 268 to 276 in 0098721

  	if (split_level > 1) {
  	body = x[(i5 + 1):(i6 - 1)]
  	h1 = grep('^<div (id="[^"]+" )?class="section level1("| )', body) + i5
  	h2 = grep('^<div (id="[^"]+" )?class="section level2("| )', body) + i5
  	h12 = setNames(c(h1, h2), rep(c('h1', 'h2'), c(length(h1), length(h2))))
  	if (length(h12) > 0 && h12[1] != i5 + 1) stop(
  	'The document must start with a first (#) or second level (##) heading'
  	)
  	h12 = sort(h12)

  
  h1 header will always be before h2 header, so if the book start with a h2 header, h12[1] != i5 + 1 will always be TRUE. I think sort(h12) should be done before the if

We may need to check the first header found according to split_by

• split_by = "chapter" => h1 should be first
• split_by = "section" => h1 or h2 could be first

Error message should be clear on what is expected.