nxml: SyntaxError: print code location; require 2-space indentation
nxml: syntax change: facilitate use of reflow in paragraphs

Previously, the following forms were allowed:

	<tag

	<tag arg=1

	<tag|some content
	  followed by some more

	<tag|closes at EOL here
	<next_tag

These forms, while sometimes handy, are problematic. When letting text
be automatically reflowed (e.g. Vim's gq), subtle errors can be
introduced requiring the user to pay attention whenever reflow happens.
It's very error prone. For instance:

	This tag is <bold|legal
	as long as it appears at the end of a line.

after reflow:

	This tag is <bold|legal as long as
	it appears at the end of a line.

Instead of just the word "legal" being bold, "legal as long as" is bold.

The syntax is updated to be more strict. Tags must always be explicitly
closed unless they use the "indented body" form.

The examples above become:
	<tag>
	<tag arg=1>
	<tag|some content
	  followed by some more>
	<tag|closes at EOL here>
	<next_tag>
	This tag is <bold|legal>
	as long as it appears at the end of a line.

This form remains legal:
	<tag arg=1
	  This is the body.