I'm starting to think that we should have the message format alongside the error code. I was looking at the list of codes and I don't quite like the fact that we have a bunch of codes there that basically doesn't mean anything unless you take one and grep the source to find the error message associated with it.

Would it be too crazy to keep the error message format in 2 places? or perhaps just keeping it in the `diag_index` and getting the format from there? The later may be a bit awkward  to be honest. It would require getting the message from the index and we would be left with something like `span_fatal!(sess, code, args)` which doesn't mean much either.

Throwing it out there... thoughts?