catch mis-spellings of session name

[jdkent]

attribute error is raised in line 518

Traceback (most recent call last):
  File "/usr/local/miniconda/bin/xnat_downloader", line 11, in <module>
    load_entry_point('xnat-downloader==0.1', 'console_scripts', 'xnat_downloader')()
  File "/usr/local/miniconda/lib/python2.7/site-packages/xnat_downloader-0.1-py2.7.egg/xnat_downloader/cli/run.py", line 667, in main
    overwrite_nii=opts.overwrite_nii)
  File "/usr/local/miniconda/lib/python2.7/site-packages/xnat_downloader-0.1-py2.7.egg/xnat_downloader/cli/run.py", line 518, in download_scan
    sub_name, ses_name = re.search(sub_ses_pattern, ses_dir).groups()
AttributeError: 'NoneType' object has no attribute 'groups'

I should catch and handle this case better, perhaps by writing to a log and then continuing along with processing.