Add a clean error when inserting objects in a table without primary key

[k4nar]

If we try to insert value in a table which doesn't have a pk, we get the following error:

(pystache) perette@bobbynette:~/git/populous/demo_superette$ populous run postgres blueprints/*.yml
Getting existing unique values...
Starting generation...
Traceback (most recent call last):
 File "/home/perette/.virtualenvs/pystache/bin/populous", line 11, in <module>
   sys.exit(cli())
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 722, in __call__
   return self.main(*args, **kwargs)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 697, in main
   rv = self.invoke(ctx)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
   return _process_result(sub_ctx.command.invoke(sub_ctx))
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
   return _process_result(sub_ctx.command.invoke(sub_ctx))
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 895, in invoke
   return ctx.invoke(self.callback, **ctx.params)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/click/core.py", line 535, in invoke
   return callback(*args, **kwargs)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/cli.py", line 75, in postgres
   db=db, user=user, password=password)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/cli.py", line 53, in _generic_run
   blueprint.generate()
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/blueprint.py", line 143, in generate
   buffer.flush()
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/buffer.py", line 23, in flush
   self.write(self.blueprint.items[item_name], buffer)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/buffer.py", line 30, in write
   item, tuple(item.db_values(obj) for obj in buffer)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/backends/postgres.py", line 76, in write
   self.get_pk_column(item.table)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/functools32/functools32.py", line 400, in wrapper
   result = user_function(*args, **kwds)
 File "/home/perette/.virtualenvs/pystache/local/lib/python2.7/site-packages/populous/backends/postgres.py", line 140, in get_pk_column
   return cursor.fetchone()[0]
TypeError: 'NoneType' object has no attribute '__getitem__'

We should return a descriptive error instead.