plugin text syntaxes: log error on cleanXHTML failure

src/plugins/plugin_misc_text_syntaxes.py

@@ -148,6 +148,10 @@ class TextSyntaxes(object):
         """
         return self.host.memory.getParamA(NAME, CATEGORY , profile_key=profile)
 
+    def _logError(self, failure, action=u"converting syntax"):
+        log.error(u"Error while {action}: {failure}".format(action=action, failure=failure))
+        return failure
+
     def cleanXHTML(self, xhtml):
         """ Clean XHTML text by removing potentially dangerous/malicious parts
         @param xhtml: raw xhtml text to clean (or lxml's HtmlElement)
@@ -191,7 +195,9 @@ class TextSyntaxes(object):
                 elt.set("style", clean_style(elt.get('style')))
             return html.tostring(xhtml_elt, encoding=unicode, method='xml')
 
-        return deferToThread(blocking_cleaning, xhtml)
+        d = deferToThread(blocking_cleaning, xhtml)
+        d.addErrback(self._logError, action=u"cleaning syntax")
+        return d
 
     def convert(self, text, syntax_from, syntax_to=_SYNTAX_XHTML, safe=True, profile=None):
         """Convert a text between two syntaxes