docs(exception): complete app.onError example with fallback for non-HTTPExceptions (#839)

Author: ramanverse

docs/api/exception.md

@@ -66,16 +66,14 @@ import { HTTPException } from 'hono/http-exception'
 
 // ...
 
-app.onError((error, c) => {
-  if (error instanceof HTTPException) {
-    console.error(error.cause)
-    // Get the custom response
-    return error.getResponse()
+app.onError((err, c) => {
+  if (err instanceof HTTPException) {
+    // Return the error response generated by HTTPException
+    return err.getResponse()
   }
-  // ...
-  // ---cut-start---
-  return c.text('Unexpected error')
-  // ---cut-end---
+  // For any other unexpected errors, log and return a generic 500 response
+  console.error(err)
+  return c.text('Internal Server Error', 500)
 })
 ```