Show "Reading xyz file" message _before_ reading Markdown or Scribble.

Previously was displayed _after_ reading the source file, but before
doing further processing.

With the change, if an error results from reading the Markdown or
Scribble source, it will be clear _what_ file.
Look for some xexprs produced by the redesigned markdown parser.

But keep looking for the old ones, just in case.

Haven't yet updated master branch on the markdown parser; just
preparing for that on the Frog end.