Add non-zero exit code to CLI for startup errors. (#553)

* Add non-zero exit code to CLI for startup errors.
* Add missing attribute to mock server.

Fixes #552.

Author: carltongibson

daphne/cli.py

@@ -289,3 +289,5 @@ def run(self, args):
             server_name=args.server_name,
         )
         self.server.run()
+        if self.server.abort_start:
+            exit(1)

tests/test_cli.py

@@ -81,6 +81,8 @@ class TestedServer:
             Mock server object for testing.
             """
 
+            abort_start = False
+
             def __init__(self, **kwargs):
                 self.init_kwargs = kwargs