Fix embedded filter silently getting ignored (#2133)

When the embedded resource is not included in the request

src/PostgREST/Error.hs

@@ -68,6 +68,7 @@ instance PgrstError ApiRequestError where
   status NoRpc{}                 = HTTP.status404
   status (UnacceptableSchema _)  = HTTP.status406
   status (ContentTypeError _)    = HTTP.status415
+  status (NotEmbedded _)         = HTTP.status400
 
   headers _ = [ContentType.toHeader CTApplicationJSON]
 
@@ -111,6 +112,9 @@ instance JSON.ToJSON ApiRequestError where
     "message" .= ("The schema must be one of the following: " <> T.intercalate ", " schemas)]
   toJSON (ContentTypeError cts)    = JSON.object [
     "message" .= ("None of these Content-Types are available: " <> T.intercalate ", " (map T.decodeUtf8 cts))]
+  toJSON (NotEmbedded resource) = JSON.object [
+    "hint"    .= ("Verify that '" <> resource <> "' is included in the 'select' query parameter." :: Text),
+    "message" .= ("Cannot apply filter because '" <> resource <> "' is not an embedded resource in this request" :: Text)]
 
 compressedRel :: Relationship -> JSON.Value
 compressedRel Relationship{..} =