[FIXED JENKINS-15806] Check return code of hg pull.

Hello, this fixed my issues with [issue 15806](https://issues.jenkins-ci.org/browse/JENKINS-15806). I did not manage to write a unit test, as the Jenkins test framework was a bit overwhelming. I would be happy to see a working unit test for this. The test i came up with (But which always fails, even in OK situation) :+1:

@Bug(15806)
public void testPullReturnCode() throws Exception
{
    File repo2 = createTmpDir();

    hg(repo, "init");
    touchAndCommit(repo, "init");
    hg(repo, "tag", "init");

    String s = repo.toString();
    hg(repo2, "clone", repo.toString(), repo2.toString());

    // delete repo 1 
    repo.delete();


    // do sample build where pull should fail
    FreeStyleProject p = createFreeStyleProject();
    MercurialSCM a = new MercurialSCM(hgInstallation(), repo2.getPath(), null, null,
            null, null, false);
    p.setScm(a);

    Future<FreeStyleBuild> b = p.scheduleBuild2(0);
    while(!b.isDone())
    {
        Thread.sleep(1000);
    }

// unfortuneately this is not correct
assertEquals(Result.SUCCESS, b.get().getResult());

}