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.

test/spec/Feature/Query/NullsStripSpec.hs

@@ -70,7 +70,7 @@ spec =
           [singularStrip]
           ""
           `shouldRespondWith`
-          [json|{"details":"The result contains 6 rows","message":"JSON object requested, multiple (or no) rows returned","code":"PGRST116","hint":null}|]
+          [json|{"details":"The result contains 6 rows","message":"Cannot coerce the result to a single JSON object","code":"PGRST116","hint":null}|]
           { matchStatus  = 406
           , matchHeaders = [matchContentTypeJson]
           }