client.py: throw error when reserved_space= is unparseable. Closes #1812.

This should now fail quickly (during "tahoe start"). Previously this
would silently treat an unparseable size as "0", and the only way to
discover that it had had a problem would be to look at the foolscap log,
or examine the storage-service web page for the unexpected "Reserved
Size" number.

src/allmydata/client.py

@@ -257,12 +257,12 @@ class Client(node.Node, pollmixin.PollMixin):
         storedir = os.path.join(self.basedir, self.STOREDIR)
 
         data = self.get_config("storage", "reserved_space", None)
-        reserved = None
         try:
             reserved = parse_abbreviated_size(data)
         except ValueError:
             log.msg("[storage]reserved_space= contains unparseable value %s"
                     % data)
+            raise
         if reserved is None:
             reserved = 0
         discard = self.get_config("storage", "debug_discard", False,