Soy errors are not being propagated

[jbalsas]

Add the following file:

`test/fixtures/src/compileError.soy

{namespace CompileError}

{template .render}
	{call Undeclared.render /}
{/template}

And the following test:

it('should exit with error when soy finishes with error', function(done) {
  console.log('running test');
      runMetal([
        'soy',
        '-s',
        'test/fixtures/src/**/CompileError.soy',
        '-d',
        'test/fixtures/src'
      ]).on('close', function(code) {
        assert.strictEqual(1, code);
        done();
      });
})

This might be caused by metal/metal-tools-soy#7