fix: clarify PGRST116 error message (#3795)

Currently it's redundant and not easy to read.

```
{"message":"JSON object requested, multiple (or no) rows returned",
"details":"The result contains 2 rows"}
```

Now:

```
{"message":"Cannot coerce the result to a single JSON object",
"details":"The result contains an array of 0 objects"}
``

Also correct docs which had a wrong error code.

docs/references/api/resource_representation.rst

@@ -74,17 +74,15 @@ This returns
 
   { "id": 1 }
 
-with a :code:`Content-Type: application/vnd.pgrst.object+json`.
-
 When a singular response is requested but no entries are found, the server responds with an error message and 406 Not Acceptable status code rather than the usual empty array and 200 status:
 
 .. code-block:: json
 
   {
-    "message": "JSON object requested, multiple (or no) rows returned",
-    "details": "Results contain 0 rows, application/vnd.pgrst.object+json requires 1 row",
-    "hint": null,
-    "code": "PGRST505"
+    "code": "PGRST116",
+    "message": "Cannot coerce the result to a single JSON object",
+    "details": "The result contains 0 rows",
+    "hint": null
   }
 
 .. note::