UI should not print traceback for missing dag/task in URL

[dimberman]

Apache Airflow version:

Kubernetes version (if you are using kubernetes) (use kubectl version):

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:
  What happened:

Right now if a user runs tries to do certain things in the UI with dags/tasks that don't exist they get confusing tracebacks rather than an error rendered in html like "the dag/task doesn't exist". One such traceback can be seen by going to the tree view for any DAG in the UI and then changing the url in the address bar for the dag_id to be a non-existent dag. The following traceback can be seen:

Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/Flask-0.10.1-py2.7.egg/flask/app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python2.7/dist-packages/Flask-0.10.1-py2.7.egg/flask/app.py", line 1477, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/local/lib/python2.7/dist-packages/Flask-0.10.1-py2.7.egg/flask/app.py", line 1381, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python2.7/dist-packages/Flask-0.10.1-py2.7.egg/flask/app.py", line 1475, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/local/lib/python2.7/dist-packages/Flask-0.10.1-py2.7.egg/flask/app.py", line 1461, in dispatch_request
return self.view_functionsrule.endpoint
File "/usr/local/lib/python2.7/dist-packages/Flask_Admin-1.4.0-py2.7.egg/flask_admin/base.py", line 68, in inner
return self._run_view(f, *args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/Flask_Admin-1.4.0-py2.7.egg/flask_admin/base.py", line 367, in _run_view
return fn(self, *args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/Flask_Login-0.2.11-py2.7.egg/flask_login.py", line 758, in decorated_view
return func(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/airflow/www/utils.py", line 213, in view_func
return f(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/airflow/www/utils.py", line 118, in wrapper
return f(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/airflow/www/views.py", line 1208, in tree
base_date = dag.latest_execution_date or datetime.now()
AttributeError: 'NoneType' object has no attribute 'latest_execution_date'

What you expected to happen:

How to reproduce it:

Anything else we need to know:

Moved here from https://issues.apache.org/jira/browse/AIRFLOW-417