make scheduler warning stack easier to parse

Summary:
Now upon dagit start:
```
/Users/prha/code/dagster/python_modules/dagit/dagit/app.py:197: UserWarning: You have defined ScheduleDefinitions for this repository, but have not defined a scheduler on the instance
  warnings.warn(MISSING_SCHEDULER_WARNING)
```

Test Plan: started dagit

Reviewers: sashank, max

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D2033

Author: prha

python_modules/dagit/dagit/app.py

@@ -34,6 +34,11 @@
 from .templates.playground import TEMPLATE as PLAYGROUND_TEMPLATE
 from .version import __version__
 
+MISSING_SCHEDULER_WARNING = (
+    'You have defined ScheduleDefinitions for this repository, but have '
+    'not defined a scheduler on the instance'
+)
+
 
 class DagsterGraphQLView(GraphQLView):
     def __init__(self, context, **kwargs):
@@ -189,10 +194,7 @@ def create_app(handle, instance, reloader=None):
                 python_path, repository_path, repository=repository, instance=instance
             )
         else:
-            warnings.warn(
-                'You have defined ScheduleDefinitions for this repository, but have '
-                'not defined a scheduler on the instance'
-            )
+            warnings.warn(MISSING_SCHEDULER_WARNING)
 
     app.add_url_rule(
         '/graphql',