Better error responses for append/prepend "not stored"

At the moment, the server only responds with "not stored" to append/prepend operations for multiple possible reasons:
-The item was over 20mb
-The item didn't exist
-others?

It would be very helpful to our users to have these split out with separate error messages. For instance, incr/decr respond with "not found" as opposed to "not stored" when the item doesn't exist, and set operations return with "value too large" when it is over 20mb.