Catch missing URL values in cost-error model method

We've encountered a few disclosure URLs that were missing some fields.
This uses the dictionary get() method, which should have been used in
the first place, to avoid key errors when running reports.

paying_for_college/models.py

@@ -400,8 +400,10 @@ def total_fields(field_list):
     def cost_error(self):
         """Return 1 or 0: Is total-cost less than tuition?"""
         url_data = self.parsed_url
-        if url_data and url_data['totl'] != '' and url_data['tuit'] != '':
-            if int(url_data['totl']) < int(url_data['tuit']):
+        if (url_data
+                and url_data.get('totl') != ''
+                and url_data.get('tuit') != ''):
+            if int(url_data.get('totl', 0)) < int(url_data.get('tuit', 0)):
                 return 1
             else:
                 return 0