Adds errors output parameter to safe procs. Refs #4.
Adds errors test. Refs #4.
Improves error template HTML code. Refs #4.
Improves docstrings. Refs #4.
Improves checking of input parameters. Refs #4.
Marks mutable variables for style. Refs #4.
Documents safe_* procs. Refs #4.
Documents task in changes log. Refs #4.
Merge branch 'feature/t4_safe_proc_return_errors'