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.

97f2079e3778511944ffb6a662520580650a3993 1.0
09c016174e332eceda015d3f43d96d7e0228acf3 1.0.1
4359ddd73b009fbb356c52ea5d99cba25222ee7a 1.1
8e621dbb82d4363a85317638ad237e2817c56347 1.1.1
093ae2915b452539b44390ee4ea14987484e1eee 1.1.2
708234ad6c97fb52417e0b46a86c8373e25123a5 1.2