mochitests: handle case where failures file is not created

                capture_exec('xvfb-run', '--server-args=-screen 0 1024x768x24',

                    './mach', $mach_command, $test->{dir});

    $test->{results}{out} = $out;

    my $failed = decode_json(scalar read_file($failures_file));

    $test->{results}{failed} = [ keys %$failed ];

    $test->{results}{success} = ! @{$test->{results}{failed}};

    my $failed = eval { -f $failures_file

                && decode_json(scalar read_file($failures_file)) };

    $test->{results}{failed} = $failed ? [ keys %$failed ] : [];

    $test->{results}{success} = $failed && ! @{$test->{results}{failed}};

}

sub build_firefox {