Internationalization (i18n) support

[Screwtapello]

There's two aspects of internationalization relevant to path_abs:

1. It should be possible for path_abs to present its own error messages (and other human-readable text) in the user's language, instead of hard-coding specific English messages.
2. path_abs should not depend on the exact text of error messages from the underlying operating system, since they can vary.

For path_abs' own error messages, the biggest offender is probably the Error::action() method which returns a string. Possibly this could be replaced with an enum, patterned after std::io::ErrorKind. There may be other issues; the code should probably be audited.

For errors from the underlying operating system, currently the only dependency I'm aware of is the test "sanity_errors", which could be amended by checking error fields individually rather than formatting the error and checking the resulting string. Again, there may be other issues and the code should probably be audited.