Add column name to ValueError from Field.clean()

It's useful to know which column failed to validate, and not always obvious.
This change adds the column name to the ValueError message, for example:

	Column 'id': invalid literal for int() with base 10: 'foo'
Merge pull request #113 from aptivate/add_column_name_to_clean_error_message

Add column name to ValueError from Field.clean()