Improve build disk use to return a success with 0 size rather than "build not found" error

[dcookspi]

Improve build disk use to return a success with 0 size rather than "build not found" error. This comes from a comment on a PR:

Though IMO it should return a success of 0 rather than "build not found" in this case. I don't consider this situation an error, and the error name doesn't fit the situation.

Originally posted by @jrray in #1280 (comment)