Assert exists before trying to calculate size; explicit elif.

If you have a path that you expect to expect to be a file, it is
confusing to see the traceback where it attempts to listdir because
self.is_file failed and the else was to treat it as a dir.
I prefer this explicit exists check, and explicit dir check.
Yeah, potential for race conditions exists.

voussoirkit/pathclass.py

@@ -248,9 +248,10 @@ class Path:
 
     @property
     def size(self):
+        self.assert_exists()
         if self.is_file:
             return os.path.getsize(self.absolute_path)
-        else:
+        elif self.is_dir:
             return sum(file.size for file in self.walk() if file.is_file)
 
     def spawn(self, path):