CHW-4607 ProxyInvalidConfig

proxy update payload failed validation; check remote_url, cache_negative_ttl_seconds, and that at least one mutable field is supplied

Default message: proxy update payload failed validation; check remote_url, cache_negative_ttl_seconds, and that at least one mutable field is supplied
HTTP status: 400 (client)

Problem. A PATCH /api/proxies/:name body failed validation. Cause. The body had an unparseable remote_url, a non-positive cache_negative_ttl_seconds, or no mutable fields. Fix. Inspect fields.field to see which key triggered the error and resubmit with a valid value. HTTP 400.