Missing project folder results in unhelpful "No git_hash specified" error

Steps to reproduce (in tor-browser-build)

1. Delete or rename the projects/clang-source folder.
2. Run ./rbm/rbm build macosx-toolchain --target release --target ncdns-osx-x86_64.

Expected results

The resulting error should convey which project is missing.

Observed results

The only error output is Error: No git_hash specified, which is rather unhelpful for figuring out which project folder is missing.