svnmeta: abort when no UUID given and none is stored on disk. Previously, not passing a UUID when instantiating an SVNMeta instance would cause it to succeed even if no UUID was previously known. First when the UUID was actually read would an exception be raised. This slight refactoring of _set_uuid() makes it so an exception is raised immediately. While at it, the exception message is changed to be slightly more accurate and helpful.

#!/bin/sh
mkdir temp || exit 1
cd temp
svnadmin create repo
svn co file://`pwd`/repo wc
pushd wc
mkdir -p project/trunk
svn add project
svn ci -m 'trunk'
cd project/trunk
echo a > a
mkdir narf
svn add a narf
svn ci -m 'file and empty dir'
popd
svnadmin dump repo > ../empty_dir_in_trunk_not_repo_root.svndump
echo 'dump in empty_dir_in_trunk_not_repo_root.svndump'
echo 'you can probably delete temp now'