tests/narrow-library.sh

py3: convert an exception message to bytes

One of our users ran into this (unrelated to my recent work on
more specific errors).

Differential Revision: https://phab.mercurial-scm.org/D9184

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF