Correct exit code when no twistd.pid file

src/wormhole/errors.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 import functools
+import click
 
 class ServerError(Exception):
     def __init__(self, message, relay):
@@ -47,7 +48,11 @@ class KeyFormatError(Exception):
 class ReflectionAttack(Exception):
     """An attacker (or bug) reflected our outgoing message back to us."""
 
-class UsageError(Exception):
+# Click needs to receive click.UsageError instances to "do the right
+# thing", which is print the error and exit -- perhaps it would be
+# better just to re-export click.UsageError here? Or use
+# click.UsageError throughout the codebase?
+class UsageError(click.UsageError, Exception):
     """The programmer did something wrong."""
 
 class WormholeClosedError(UsageError):

src/wormhole/server/cmd_server.py

@@ -1,5 +1,6 @@
 from __future__ import print_function, unicode_literals
 import os, time
+from ..errors import UsageError
 from twisted.python import usage
 from twisted.scripts import twistd
 
@@ -38,8 +39,9 @@ def kill_server():
     try:
         f = open("twistd.pid", "r")
     except EnvironmentError:
-        print("Unable to find twistd.pid . Is this really a server directory?")
-        return
+        raise UsageError(
+            "Unable to find 'twistd.pid' -- is this really a server directory?"
+        )
     pid = int(f.read().strip())
     f.close()
     os.kill(pid, 15)