Add 500 error (#190)

fern/pages/cohere-api/errors.mdx

@@ -154,3 +154,7 @@ To resolve this error, consult [the API spec](https://docs.cohere.com/reference/
     |failed to get rerank inference: request cancelled|
     |request cancelled by user|
 </details>
+
+## 500 - Server Error
+
+500 responses are sent when there is an unexpected internal server error. To resolve these errors, please contact support via [email](mailto:[email protected]) or [discord](https://discord.gg/XW44jPfYJu) with details about your request and use case.