Always respond with data if it exists

[Xophmeister]

If a range request is made and that section of the data is available (even if it is still being fetched from iRODS or having its checksums calculated), then that data should be returned rather than issuing a "please wait"-style response.

We need to decide on how to settle the edge-case of a partial range existing. Do you:

• Flaunt the request and only return the data that exists?
• Chunk the response into the data that exists and an ETA for the remaining range?
• Return the data that exists, then pause/stream the remainder as it becomes available?
• Fallback to returning nothing but the "please wait" response?

I'd go with the second option...