Add Errors reference (#430)

* Adds PostgREST' error codes 

* Move error information to a dedicated reference page

* Organize errors into tables

docs/tutorials/tut0.rst

@@ -223,7 +223,7 @@ Response is 401 Unauthorized:
     "hint": null,
     "details": null,
     "code": "42501",
-    "message": "permission denied for relation todos"
+    "message": "permission denied for table todos"
   }
 
 There we have it, a basic API on top of the database! In the next tutorials we will see how to extend the example with more sophisticated user access controls, and more tables and queries.

docs/tutorials/tut1.rst

@@ -175,7 +175,12 @@ After expiration, the API returns HTTP 401 Unauthorized:
 
 .. code-block:: json
 
-  {"message":"JWT expired"}
+  {
+    "hint": null,
+    "details": null,
+    "code": "PGRST301",
+    "message": "JWT expired"
+  }
 
 Bonus Topic: Immediate Revocation
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