docs: add docs for error handling (fixes #317)

PiperOrigin-RevId: 726463667

README.md

@@ -1103,4 +1103,19 @@ print(async_pager[0])
 delete_job = client.batches.delete(name=job.name)
 
 delete_job
+```
+
+## Error Handling
+
+To handle errors raised by the model service, the SDK provides this [APIError](https://github.com/googleapis/python-genai/blob/main/google/genai/errors.py) class.
+
+```python
+try:
+  client.models.generate_content(
+      model="invalid-model-name",
+      contents="What is your name?",
+  )
+except errors.APIError as e:
+  print(e.code) # 404
+  print(e.message)
 ```