Add "warmup mode" for RPC server.

Start the RPC server before doing all the (expensive) startup initialisations like loading the block index.  Until the node is ready, return all calls immediately with a new error signalling "in warmup" with an appropriate status message (similar to the init message).

This is useful for RPC clients to know that the server is there (e. g., they don't have to start it) but not yet available.  It is used in Namecoin and Huntercoin already for some time, and there exists a UI hooked onto the RPC interface that actively uses this to its advantage.
Yes I do think it is better to do it that way. The translation is a bit of a red herring - for bitcoind _() is a no-op. But in general If you intend this to be machine-readable, you need a status byte/word. A message can be changed at any time and should not be used for that.