Summary:
1. One call to `Core.Std.String.slice` was wrong and caused the program to crash, and
2. The crash was silently ignored because the error code of uncaught OCaml
exceptions was the same as `CheckCopyright.copyright_malformed_exit_code` (=2)
Address both issues. Also build CheckCopyright with debug options.
Reviewed By: jberdine
Differential Revision: D4410306
fbshipit-source-id: d73b086