revlog: avoid raising no-arg RevlogError for internal flow control
I'm about to make RevlogError require a `message` argument and this
code was failing. This patch refactors it to not raise an exception
for intra-function flow control.
Differential Revision: https://phab.mercurial-scm.org/D10740
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh="$PYTHON" "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF