I found another case that results in an 'executable not found' error.
This happens when I create a new project in appCode.
Some investigation shows that this seems to be due to the source files created by the template not being made part of the executable target, so that what's actually being built is a product without any sources, which doesn't work (but also doesn't report an error).
Jumping the project over to Xcode to assign the source files to the build targets fixes the issue, after which I can continue working in appCode.
I've raised OC-624 with (overly) detailed steps to reproduce the issue.