Workflow import should warn if the status does not exist

If you import an XML workflow that came from another instance, without making sure that all the issue status existed in the new instance, there will be probems:

http://confluence.atlassian.com/display/JIRAKB/NullPointerException+after+importing+workflow+XML

It would be better if the import function would warn with an error message saying that the XML import was referencing unexisting objects.