Adding exception handler support - fix tests + docs

Author: leandrodamascena

docs/core/event_handler/appsync.md

@@ -288,6 +288,19 @@ You can use `append_context` when you want to share data between your App and Ro
     --8<-- "examples/event_handler_graphql/src/split_operation_append_context_module.py"
 	```
 
+### Exception handling
+
+You can use **`exception_handler`** decorator with any Python exception. This allows you to handle a common exception outside your resolver, for example validation errors.
+
+The `exception_handler` function also supports passing a list of exception types you wish to handle with one handler.
+
+```python hl_lines="5-7 11" title="Exception handling"
+--8<-- "examples/event_handler_graphql/src/exception_handling_graphql.py"
+```
+
+???+ warning
+    This is not supported when using async single resolvers.
+
 ### Batch processing
 
 ```mermaid

examples/event_handler_graphql/src/exception_handling_graphql.py

@@ -0,0 +1,17 @@
+from aws_lambda_powertools.event_handler import AppSyncResolver
+
+app = AppSyncResolver()
+
+
+@app.exception_handler(ValueError)
+def handle_value_error(ex: ValueError):
+    return {"message": "error"}
+
+
+@app.resolver(field_name="createSomething")
+def create_something():
+    raise ValueError("Raising an exception")
+
+
+def lambda_handler(event, context):
+    return app.resolve(event, context)