webhook_decorator: Support notifying bot owner on invalid JSON.

Our webhook-errors.log file is riddled with exceptions that are
logged when a webhook is incorrectly configured to send data in
a non-JSON format. To avoid this, api_key_only_webhook_view
now supports an additional argument, notify_bot_owner_on_invalid_json.
This argument, when True, will send a PM notification to the bot's
owner notifying them of the configuration issue.

zerver/lib/exceptions.py

@@ -28,6 +28,7 @@ class ErrorCode(AbstractEnum):
     BAD_REQUEST = ()  # Generic name, from the name of HTTP 400.
     REQUEST_VARIABLE_MISSING = ()
     REQUEST_VARIABLE_INVALID = ()
+    INVALID_JSON = ()
     BAD_IMAGE = ()
     REALM_UPLOAD_QUOTA = ()
     BAD_NARROW = ()
@@ -142,5 +143,12 @@ class RateLimited(PermissionDenied):
     def __init__(self, msg: str="") -> None:
         super().__init__(msg)
 
+class InvalidJSONError(JsonableError):
+    code = ErrorCode.INVALID_JSON
+
+    @staticmethod
+    def msg_format() -> str:
+        return _("Malformed JSON")
+
 class BugdownRenderingException(Exception):
     pass