add docs for exception handler

docs/utility_methods/exception_handlers.md

@@ -0,0 +1,42 @@
+# Exception handlers
+
+_Added in v0.5.7_
+
+Exceptions handlers are functions that can be assigned to your app to handle exceptions that occur during the application runtime.
+They are useful for customizing the response when an error occurs, logging errors, and performing cleanup tasks.
+
+## Types
+
+Reflex support two type of exception handlers `frontend_exception_handler` and `backend_exception_handler`.
+
+They are used to handle exceptions that occur in the `frontend` and `backend` respectively.
+
+The `frontend` errors are coming from the JavaScript side of the application, while `backend` errors are coming from the the event handlers on the Python side.
+
+## Register an Exception Handler
+
+To register an exception handler, assign it to `app.frontend_exception_handler` or `app.backend_exception_handler` to assign a function that will handle the exception.
+
+The expected signature for an error handler is `def handler(exception: Exception)`.
+
+```md alert warning
+# Only named functions are supported as exception handler.
+```
+
+## Examples
+
+```python
+import reflex as rx
+
+def custom_frontend_handler(exception: Exception) -> None:
+    # My custom logic for frontend errors
+    pass
+
+def custom_backend_handler(exception: Exception) -> rx.event.EventSpec:
+    # My custom logic for backend errors
+    pass
+
+app = rx.App()
+app.frontend_exception_handler = custom_frontend_handler
+app.backend_exception_handler = custom_backend_handler
+```