suggest to update PySM 3 in case file not found

[zonca]

See #164

[zonca]

Now if a template is not found you get the error:

E               urllib.error.URLError: <urlopen error Unable to open any source! Exceptions were {'https://portal.nersc.gov/project/cmb/pysm-data/doesntexist.txt': <HTTPError 404: 'Not Found'>, 'http://www.astropy.org/astropy-data/doesntexist.txt': <HTTPError 404: 'Not Found'>}>

/usr/lib/python3/dist-packages/astropy/utils/data.py:1259: URLError
----------------------------------------------------------------------------------------- Captured log call ------------------------------------------------------------------------------------------
ERROR    pysm3:data.py:54 File not found, please make sure you are using the latest version of PySM 3