Fix an error with an initial Boundary-Line import without --commit

Thanks to Geoff Stockham for pointing out this problem.  This
fixes issue #48.

When we see a second polygon with the same unit_id and this
is a new import, a dry-run import will fail because the name
can't be found in the database from the unit_id - it was never
saved the first time around.  As with finding the name from
ons_code we should ignore this error if we're not saving to the
database.

(The example in the bug report is the second polygon in
Waterbeach ED in county_electoral_division_region.shp.)

mapit/management/commands/mapit_UK_import_boundary_line.py

@@ -71,7 +71,10 @@ def handle_label(self,  filename, **options):
 
             if unit_id in self.unit_id_to_shape:
                 m, poly = self.unit_id_to_shape[unit_id]
-                m_name = m.names.get(type=name_type).name
+                try:
+                    m_name = m.names.get(type=name_type).name
+                except Name.DoesNotExist:
+                    m_name = m.name # If running without commit for dry run, so nothing being stored in db
                 if name != m_name:
                     raise Exception, "Unit ID code %s is used for %s and %s" % (unit_id, name, m_name)
                 # Otherwise, combine the two shapes for one area