esg_listdatasets can error if datasets publication is interrupted

[sterlingbaldwin]

If a dataset publication is interrupted part way, and a user runs esg_listdatasets, they can get the following error:

(esgf-pub) -bash-4.1$ esglist_datasets e3sm | grep E3SM.1_0.historical
/usr/local/conda/envs/esgf-pub/lib/python2.7/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
Traceback (most recent call last):
  File "/usr/local/conda/envs/esgf-pub/bin/esglist_datasets", line 143, in <module>
    main(sys.argv[1:])
  File "/usr/local/conda/envs/esgf-pub/bin/esglist_datasets", line 139, in main
    result, headers = queryDatasets(projectName, handler, Session, properties, select=selectList, listall=listAll)
  File "/usr/local/conda/envs/esgf-pub/lib/python2.7/site-packages/esgcet/query/query.py", line 312, in queryDatasets
    result = [(item, item.getLatestVersion()) for item in result1]
  File "/usr/local/conda/envs/esgf-pub/lib/python2.7/site-packages/esgcet/model/__init__.py", line 718, in getLatestVersion
    return self.versions[-1]
IndexError: list index out of range