an exception. Updated all callers.

Rev: src/client.c:1.166.2.2
Rev: src/io.c:1.215.2.4
Rev: src/io.h:1.110.2.2
Rev: src/lsh-decode-key.c:1.22.2.1
Rev: src/lsh-decrypt-key.c:1.2.2.1
Rev: src/lsh-export-key.c:1.20.2.1
Rev: src/lsh-keygen.c:1.20.2.1
Rev: src/lsh-make-seed.c:1.15.2.1
Rev: src/lsh-writekey.c:1.40.2.1
Rev: src/unix_interact.c:1.27.2.1
Rev: src/unix_random.c:1.17.2.1
Rev: src/werror.c:1.73.2.2