Produce error message, not traceback, when test program doesn't build.

A traceback from a Python script always makes people think there's a bug in
your script. Even when a test program fails to build, CMake often (always?)
tries to run it anyway, via our run_build_test.py script. For that case,
produce a straightforward error message -- rather than an OSError traceback
that doesn't even mention the program name!