fix(errors): add res param to onError hook and guard against internal throw

- onError(err, req, res): response object now available for observability
- Wrap onError call in try/catch — hook failure must not suppress original error
- Type definition updated: onError?: (error: Error, request: Request, response: Response) => void

Closes #101

Author: molty3000

index.d.ts

@@ -59,6 +59,7 @@ declare namespace fastgateway {
     onRequest?: Function
     rewriteHeaders?: Function
     onResponse?: Function
+    onError?: (error: Error, request: Request, response: Response) => void
     rewriteRequestHeaders?: Function
     request?: {
       timeout?: number

index.js

@@ -173,6 +173,15 @@ const handler = (route, proxy, proxyHandler) => async (req, res, next) => {
       proxyHandler(req, res, req.url, proxy, proxyOpts)
     }
   } catch (err) {
+    const { onError } = hooks
+    if (typeof onError === 'function') {
+      try {
+        onError(err, req, res)
+      } catch (_) {
+        // onError hook failure must not suppress the original error
+      }
+    }
+
     return next(err)
   }
 }