Refactoring of storage code:

1. Fix some bugs in local_storage
2. Refactor unit tests to not set Media.key explicitly (unless it's well-known beforehand, which it isn't)
3. Limit length of URL for 'url' type path_generator
4. Throw an error if 'save_to' of local storage is too long
5. A few other tidyups

src/auto_archiver/core/consts.py

@@ -1,3 +1,5 @@
+class SetupError(ValueError):
+    pass
 
 MODULE_TYPES = [
     'feeder',