Add useful message to the ParseError exception.

Sometimes, I would like to be able to catch ParseError, and output its
message immediately.  Set the exception's message to the parser error
message.

html5lib/html5parser.py

@@ -18,6 +18,7 @@
 from .constants import tokenTypes, ReparseException, namespaces
 from .constants import htmlIntegrationPointElements, mathmlTextIntegrationPointElements
 from .constants import adjustForeignAttributes as adjustForeignAttributesMap
+from .constants import E
 
 
 def parse(doc, treebuilder="etree", encoding=None,
@@ -256,7 +257,7 @@ def parseError(self, errorcode="XXX-undefined-error", datavars={}):
  # XXX The idea is to make errorcode mandatory.
  self.errors.append((self.tokenizer.stream.position(), errorcode, datavars))
  if self.strict:
- raise ParseError
+ raise ParseError(E[errorcode] % datavars)
 
  def normalizeToken(self, token):
  """ HTML5 specific normalizations to the token stream """