Issue warning for duplicate header identifiers.

As noted in the previous commit, an autogenerated identifier
may still coincide with an explicit identifier that is given
for a header later in the document, or with an identifier on
a div, span, link, or image. This commit adds a warning
in this case, so users can supply an explicit identifier.

* Added `DuplicateIdentifier` to LogMessage.
* Modified HTML, Org, MediaWiki readers so their custom
  state type is an instance of HasLogMessages.  This is necessary
  for `registerHeader` to issue warnings.

See #1745.

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