Improve a couple of sql/json error messages

Fix the grammar in two, and add a hint to one.

Author: adunstan

src/backend/parser/parse_expr.c

@@ -4456,7 +4456,7 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			if (jsexpr->returning->typid != JSONBOID)
 				ereport(ERROR,
 						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("JSON_TABLE() is not yet implemented for json type"),
+						 errmsg("JSON_TABLE() is not yet implemented for the json type"),
 						 errhint("Try casting the argument to jsonb"),
 						 parser_errposition(pstate, func->location)));
 
@@ -4466,7 +4466,8 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 	if (exprType(contextItemExpr) != JSONBOID)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("%s() is not yet implemented for json type", func_name),
+				 errmsg("%s() is not yet implemented for the json type", func_name),
+				 errhint("Try casting the argument to jsonb"),
 				 parser_errposition(pstate, func->location)));
 
 	return (Node *) jsexpr;

src/test/regress/expected/json_sqljson.out

@@ -1,21 +1,24 @@
 -- JSON_EXISTS
 SELECT JSON_EXISTS(NULL FORMAT JSON, '$');
-ERROR:  JSON_EXISTS() is not yet implemented for json type
+ERROR:  JSON_EXISTS() is not yet implemented for the json type
 LINE 1: SELECT JSON_EXISTS(NULL FORMAT JSON, '$');
                ^
+HINT:  Try casting the argument to jsonb
 -- JSON_VALUE
 SELECT JSON_VALUE(NULL FORMAT JSON, '$');
-ERROR:  JSON_VALUE() is not yet implemented for json type
+ERROR:  JSON_VALUE() is not yet implemented for the json type
 LINE 1: SELECT JSON_VALUE(NULL FORMAT JSON, '$');
                ^
+HINT:  Try casting the argument to jsonb
 -- JSON_QUERY
 SELECT JSON_QUERY(NULL FORMAT JSON, '$');
-ERROR:  JSON_QUERY() is not yet implemented for json type
+ERROR:  JSON_QUERY() is not yet implemented for the json type
 LINE 1: SELECT JSON_QUERY(NULL FORMAT JSON, '$');
                ^
+HINT:  Try casting the argument to jsonb
 -- JSON_TABLE
 SELECT * FROM JSON_TABLE(NULL FORMAT JSON, '$' COLUMNS (foo text));
-ERROR:  JSON_TABLE() is not yet implemented for json type
+ERROR:  JSON_TABLE() is not yet implemented for the json type
 LINE 1: SELECT * FROM JSON_TABLE(NULL FORMAT JSON, '$' COLUMNS (foo ...
                                  ^
 HINT:  Try casting the argument to jsonb