[MNG-5075] MavenProject.getParent throws undocumented ISE

Fix for [MNG-5075](http://jira.codehaus.org/browse/MNG-5075): `MavenProject.getParent` throws `IllegalStateException` when there is a model building error. Patch (updated version of JIRA original) just sends any problems to the logger and returns `null`, which is not great but is at least now documented and will prevent innocent clients from being fatally broken by an unchecked exception.

I am `jglick@apache.org` and intend this patch to be submitted to the ASF under the ASL 2.0.