Exit from build if folder doesn't exist

Summary:
In normal circumstances, bootstrap.sh should create a build folder.

If it fails, but returns a 0 exit code, or if a concurrent process
cleans it, we now detect the odd case and exit.

Test Plan: shellcheck roles/shellserver/odderon/files/build.sh

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D942