botserver: Make bot-name-not-found error more user-friendly.

Previously, when a bot name wasn't found, the Botserver threw
an ImportError with an error message. This results in an
intimidating traceback which is in most cases not helpful to
the user. This commit replaces the ImportError with sys.exit.

zulip_botserver/tests/test_server.py

@@ -3,7 +3,6 @@ import os
 from typing import Any, Dict
 import unittest
 from .server_test_lib import BotServerTestCase
-import six
 import json
 
 from zulip_botserver import server
@@ -108,19 +107,19 @@ class BotServerTests(BotServerTestCase):
                 'token': 'abcd1234',
             }
         }
-        # TODO: The following passes mypy, though the six stubs don't match the
-        # unittest ones, so we could file a mypy bug to improve this.
-        six.assertRaisesRegex(self,
-                              ImportError,
-                              "Bot \"nonexistent-bot\" doesn't exists. Please "
-                              "make sure you have set up the botserverrc file correctly.",
-                              lambda: self.assert_bot_server_response(
+        # This works, but mypy still complains:
+        # error: No overload variant of "assertRaisesRegexp" of "TestCase" matches argument types
+        # [def (*args: builtins.object, **kwargs: builtins.object) -> builtins.SystemExit, builtins.str]
+        with self.assertRaisesRegexp(SystemExit,  # type: ignore
+                                     'Error: Bot "nonexistent-bot" doesn\'t exist. Please make '
+                                     'sure you have set up the botserverrc file correctly.'):
+            self.assert_bot_server_response(
                 available_bots=available_bots,
                 event=dict(message={'content': "@**test** test message"},
                            bot_email='helloworld-bot@zulip.com',
                            trigger='mention',
                            token='abcd1234'),
-                                  bots_config=bots_config))
+                bots_config=bots_config)
 
     @mock.patch('sys.argv', ['zulip-botserver', '--config-file', '/foo/bar/baz.conf'])
     def test_argument_parsing_defaults(self) -> None:

zulip_botserver/zulip_botserver/server.py

@@ -57,8 +57,8 @@ def load_lib_modules(available_bots: List[str]) -> Dict[str, Any]:
             lib_module = import_module(module_name)
             bots_lib_module[bot] = lib_module
         except ImportError:
-            raise ImportError(
-                "\nImport Error: Bot \"{}\" doesn't exists. "
+            sys.exit(
+                "Error: Bot \"{}\" doesn't exist. "
                 "Please make sure you have set up the botserverrc file correctly.\n".format(bot)
             )
     return bots_lib_module