Error message formatting

When used in an application, the library generates strings which are not easily integrated into application error messages. Adding punctuation or newlines is always easy from the calling code. Therefore I propose these simplifications, which closer match the standard UNIX error reporting conventions.

This branch depends on the "cleanup" branch, for which Pull Request #9 was opened. I'm not sure if GitHub will correctly handle this dependency. If this poses a problem, please ask me to rebase onto master.