Description of problem:

The use of <errortext>, <errorname>, etc., currently produce build warnings in publican.

Version-Release number of selected component (if applicable):
publican-2.8-1.el6eng

How reproducible:
always

Steps to Reproduce:
1. build a book that uses <errortext>
2.
3.
  
Actual results:
*WARNING: Unvalidated tag: 'errortext'. This tag may not be displayed correctly, may generate invalid xhtml, or may breach Section 508 Accessibility standards.

Expected results:
Unless there is a reason not to, this tag should be included in the list of known tags (with or without special formatting?).

Additional info:
Text included within <errortext> tags currently appears the same as the surrounding body text (to be expected). There doesn't appear to be any issue with the tag itself.