R1:764d3d1c6543

Make "pcntl" script error more explicit

Summary: This may make it easier to debug problems with CLI + pcntl

Test Plan: Added a syntax error to the script and got more useful output

Reviewers: johnduhart, jungejason, tuomaspelkonen, aran

Reviewed By: johnduhart

CC: aran, johnduhart

Differential Revision: 869