plugins: added new MissingModule exceptions to plugins using third party modules

src/plugins/plugin_xep_0071.py

@@ -25,7 +25,10 @@ log = getLogger(__name__)
 from wokkel import disco, iwokkel
 from zope.interface import implements
 # from lxml import etree
-from lxml import html
+try:
+    from lxml import html
+except ImportError:
+    raise exceptions.MissingModule(u"Missing module lxml, please download/install it from http://lxml.de/")
 try:
     from twisted.words.protocols.xmlstream import XMPPHandler
 except ImportError: