more strict about matchnig only what's legal.  The expression ends
    up being a bit more complex, and needs additional checks to be
    done on what follows.

HTMLParser.check_for_whole_start_tag():  Helper method that uses
    locatestarttagend, performs the required additional checks, and
    determines whether we've actually found the end of the start tag,
    are at a buffer boundary, or have encountered an syntactical
    error.


HTMLParser.parse_starttag():  Use check_for_whole_start_tag() to see
    if we really have the start tag.

HTMLParseError.__init__():  Simplify assertion.


This should close ZPT(18).