Add better error message if duplicate external IDs exist

[stuppie]

Currently get

Traceback (most recent call last):
 File "/home/nuria/workspace/comm2net/Krusty/wd_to_neo4j.py", line 146, in <module>
   main(**d)
 File "/home/nuria/workspace/comm2net/Krusty/wd_to_neo4j.py", line 132, in main
   bot.run()
 File "/home/nuria/workspace/comm2net/Krusty/wd_to_neo4j.py", line 118, in run
   edge_lines.extend(self.handle_statement(s, start_id))
 File "/home/nuria/workspace/comm2net/Krusty/wd_to_neo4j.py", line 95, in handle_statement
   line[':END_ID'] = self.qid_dbxref["Q" + str(s.get_value())] if s.data_type == "wikibase-item" else s.get_value()
KeyError: 'Q184'