zcommand: Raise error when command doesn't begin with a slash.

zerver/lib/zcommand.py

@@ -6,7 +6,9 @@ from zerver.lib.actions import do_set_user_display_setting
 from zerver.lib.exceptions import JsonableError
 
 def process_zcommands(content: str, user_profile: UserProfile) -> Dict[str, Any]:
-    command = content.replace('/', '')
+    if not content.startswith('/'):
+        raise JsonableError(_('There should be a leading slash in the zcommand.'))
+    command = content[1:]
 
     if command == 'ping':
         ret = dict()  # type: Dict[str, Any]