add docstring and replace func name by a good one.
1. follow PEP-257 add docstring.
2. change func naem "task_result_to_html_or_string" to "task_result_to_html".
3. add postfix instance attribute for TaskResultLine.
4. add empty_str_placeholder instance attribute for TaskResultFragment.
5. recover class NoSuchException.
6. honoring line limit for dictionaries.