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.

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
    + Fixed `"column reference <col> is ambiguous"` error when selecting `?select=...table(col,count())`
    + Fixed `"column <json_aggregate>.<alias> does not exist"` error when selecting `?select=...table(aias:count())`
  - #3727, Clarify "listening" logs - @steve-chavez
+ - #3795, Clarify `Accept: vnd.pgrst.object` error message - @steve-chavez
  - #3779, Always log the schema cache load time - @steve-chavez
 
 ### Changed