utils: improve error output when tsig fails.

src/libknot/errcode.c

src/libknot/internal/errcode.h

src/libknot/rrtype/tsig.c

src/utils/common/msg.h

src/utils/common/sign.c

src/utils/kdig/kdig_exec.c

src/utils/knsupdate/knsupdate_exec.c