[MM-61079] Fix error checking in webhook.go (#30611)

* MM-28751 Fix error checking in webhook.go

- Implement proper error checking for r.ParseForm()
- Implement proper error checking for r.ParseMultipartForm()
- Implement proper error checking for w.Write()
- Remove webhook.go exception from .golangci.yml

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Co-Authored-By: Claude <noreply@anthropic.com>

* Fix translation

---------

Co-authored-by: Claude <noreply@anthropic.com>

server/i18n/en.json

@@ -10544,6 +10544,14 @@
     "id": "web.incoming_webhook.parse.app_error",
     "translation": "Unable to parse incoming data."
   },
+  {
+    "id": "web.incoming_webhook.parse_form.app_error",
+    "translation": "Failed to parse form for webhook {{.hook_id}}."
+  },
+  {
+    "id": "web.incoming_webhook.parse_multipart.app_error",
+    "translation": "Failed to parse multipart form for webhook {{.hook_id}}."
+  },
   {
     "id": "web.incoming_webhook.permissions.app_error",
     "translation": "User {{.user}} does not have appropriate permissions to channel {{.channel}}"