Ensure errors from build script are raised

The webassets script will return a numeric error value in certain
failure cases (such as FilterErrors raised by some filters) instead of
raising an exception. Since run_with_argv is called directly here, we
need to check its return value and raise a Django CommandError if the
build command indicated a failure.
"Merge pull request #29 from wyattanderson/master\n\nEnsure errors from build script are raised"