Fixed miessage_substitutions in missing_header (#340)

* Fixed miessage_substitutions im missing_header

* Improved an error message

* Fixed linting

goodtables/checks/missing_header.py

@@ -22,12 +22,17 @@ def missing_header(cells, sample):
             continue
 
         # Add error
-        message_substitutions = {
-            'field_name': '"{}"'.format(cell['field'].name),
-        }
+        field_name = cell['field'].name if cell['field'] else ''
+        message_substitutions = {'field_name': '"{}"'.format(field_name)}
+        message = 'There is a missing header in column {column_number}'
+        # It's a temporary solution for
+        # https://github.com/frictionlessdata/goodtables-py/issues/338
+        if not cell.get('column-number'):
+            message = 'There is a missing header in column {field_name}'
         error = Error(
             'missing-header',
             cell,
+            message=message,
             message_substitutions=message_substitutions
         )
         errors.append(error)