ambiguous exception needs to be more specific

[jjmanzer]

Describe the bug
When an assignment_group is provided to create_change_request() that is invalid, we get an unclear message back leaving the user (and dev) unsure of what to do.

To Reproduce
Steps to reproduce the behavior:

change_order = ChangeRequestHandler().create_change_request(
    co_title,
    co_description,
    assignment_group="bad_assignment_group",
    payload=payload
)

Expected behavior
A clear and concise message like "your assignment_group XXX does not exist".

Additional context
Exception text

Traceback (most recent call last):
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/pysnow/response.py", line 173, in one
result = next(r)
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/celery/app/trace.py", line 382, in trace_task
R = retval = fun(*args, **kwargs)
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/newrelic/hooks/application_celery.py", line 85, in wrapper
return wrapped(*args, **kwargs)
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/celery/app/trace.py", line 641, in __protected_call__
return self.run(*args, **kwargs)
File "/usr/local/share/thor_api/netexec/tasks.py", line 59, in execute_request
request_handler.run()
File "/usr/local/share/thor_api/netexec/handlers/base.py", line 58, in run
if self.create_automated_change_order():
File "/usr/local/share/thor_api/netexec/handlers/base.py", line 109, in create_automated_change_order
payload=payload
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/django_snow/helpers/snow_request_handler.py", line 47, in create_change_request
payload['assignment_group'] = self.get_snow_group_guid(assignment_group or self.snow_assignment_group)
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/django_snow/helpers/snow_request_handler.py", line 147, in get_snow_group_guid
result = response.one()
File "/usr/local/share/thor_api/.venv/lib/python3.6/site-packages/pysnow/response.py", line 175, in one
raise NoResults("No records found")
pysnow.exceptions.NoResults: No records found