Fix problem with partial schemas

datapackage/resource.py

@@ -8,6 +8,7 @@
 import json
 import six
 import six.moves.urllib as urllib
+from future.utils import raise_with_traceback
 import tabulator
 import jsontableschema
 
@@ -332,15 +333,17 @@ def __iter__(self):
 
     def __next__(self):
         row = next(self._tabulator_iter)
-        row_dict = {}
+        row = dict(zip(row.headers,row.values))
         if self._schema is not None:
-            for index, field in enumerate(self._schema.fields):
-                value = row.values[index]
-                value = self._schema.cast(field['name'], value)
-                row_dict[field['name']] = value
-        else:
-            row_dict = dict(zip(row.headers, row.values))
-        return row_dict
+            for field in self._schema.fields:
+                field_name = field['name']
+                value = row[field_name]
+                try:
+                    value = self._schema.cast(field_name, value)
+                except Exception as exception:
+                    raise_with_traceback(ValueError('Cannot cast %r for <%s>' % (value, field_name)))
+                row[field_name] = value
+        return row
 
     def next(self):
         # For Py27 compatibility