Provide reasonable error message when accessing grade data non-existent flow

A scenario in which this can happen is

* Preview branch with newly created flow
* Switch back to master branch
* Whoops, flow is gone

Currently, this results in a server error with this traceback:

```
File "./course/content.py" in get_repo_blob
268. mode, blob_sha = access_directory_content(tree, names[-1])

File "./course/content.py" in access_directory_content
244. mode_and_blob_sha = maybe_tree[name.encode()]

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/dulwich/objects.py" in __getitem__
936. return self._entries[name]

During handling of the above exception (b'exam_1.yml'), another exception occurred:

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
35. response = get_response(request)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
128. response = self.process_exception_by_middleware(e, request)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
126. response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "./course/utils.py" in wrapper
792. response = f(pctx, *args, **kwargs)

File "./course/grading.py" in grade_flow_page
154. pctx.course.identifier, respect_preview=False)

File "./course/flow.py" in adjust_flow_session_page_data
301. flow_session.flow_id, commit_sha)

File "./course/content.py" in get_flow_desc
1397. flow_desc = get_yaml_from_repo(repo, "flows/%s.yml" % flow_id, commit_sha)

File "./course/content.py" in get_yaml_from_repo
620. repo, full_name, commit_sha, allow_tree=False).data

File "./course/content.py" in get_repo_blob
278. raise ObjectDoesNotExist(_("resource '%s' not found") % full_name)

Exception Type: ObjectDoesNotExist at /course/cs450-f18/grading/flow-page/428358/2/
Exception Value: resource 'flows/exam_1.yml' not found
```

Start in course/content.py at get_repo_blob and follow the call path through get_yaml_from_repo, course/flow.py, and course/grading.py. Reproduce the missing-flow scenario after switching branches, then verify that accessing the absent flow produces a reasonable user-facing error instead of the shown server traceback.

Written by the indexing model from the issue text.