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/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