dust grid choice exceptions

[dnarayanan]

when choosing a dust grid choice outside of the allowed values, one ought to get a valueerror per grid_construction.py:

       dust_grid_type_list = ['dtm', 'rr', 'manual','li_bestfit']
        try:
            dust_choice = dust_grid_type_list.index(cfg.par.dust_grid_type)
        except ValueError as e:
            print('You chose a dust_choice that isnt a valid selection within the list: ' +
                  dust_grid_type_list+'....crashing now!')
            sys.exit()

but instead it errors with something like:

Smallest Cell Edge:  0.01220703125000022 kpc
Smallest Cell Volume:  1.8189894035459552e-06 kpc**3
Biggest Cell Edge:  12.500000000000226 kpc
Biggest Cell Volume:  1953.125000000106 kpc**3
Left Edge:  40183.377386093845 kpc
Right Edge:  40383.377386093845 kpc
Bottom Edge:  31824.019815372692 kpc
Top Edge:  32024.019815372696 kpc
Nearest Edge:  40834.63570342197 kpc
Farthest Edge:  32024.019815372696 kpc
Traceback (most recent call last):
  File "/home/desika.narayanan/miniconda3/envs/mypy2env/bin/pd_front_end.py", line 4, in <module>
    __import__('pkg_resources').run_script('powderday==0.1.0', 'pd_front_end.py')
  File "/home/desika.narayanan/miniconda3/envs/mypy2env/lib/python2.7/site-packages/pkg_resources/__init__.py", line 666, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/home/desika.narayanan/miniconda3/envs/mypy2env/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1460, in run_script
    exec(script_code, namespace, namespace)
  File "/home/desika.narayanan/miniconda3/envs/mypy2env/lib/python2.7/site-packages/powderday-0.1.0-py2.7.egg/EGG-INFO/scripts/pd_front_end.py", line 70, in <module>

  File "build/bdist.linux-x86_64/egg/powderday/sph_tributary.py", line 23, in sph_m_gen
  File "build/bdist.linux-x86_64/egg/powderday/grid_construction.py", line 94, in yt_octree_generate
TypeError: cannot concatenate 'str' and 'list' objects