Warning/error function refactoring

[Sylvie]

src/error.cpp defines warn, warnx, err, and errx. We only use errx. All warnings are currently manually coding their "warning: " prefix; we should use warnx. Also, instead of the err variant, we manually pass strerror(errno) (if that's technically necessary, then we should remove err and warn as useless).

[Sylvie]

This was a misunderstanding due to code coverage output that hadn't triggered warnings/errors.

[Eldred Habert]

Yeah, some of the formatting is inconsistent, especially across tools; but I intend to have that fixed in rsgbds by making all of that common anyway.