Avoid splitting errmsg string to span multiple lines

The error message being fixed was added in 6185c9737c.

While at it, add an "a" to the sentence.

Reported-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20240322.095149.895185546948714852.horikyota.ntt%40gmail.com

src/backend/parser/parse_expr.c

@@ -4586,9 +4586,7 @@ transformJsonBehavior(ParseState *pstate, JsonBehavior *behavior,
 				!IsA(expr, OpExpr))
 				ereport(ERROR,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
-						 errmsg("can only specify constant, non-aggregate"
+						 errmsg("can only specify a constant, non-aggregate function, or operator expression for DEFAULT"),
-								" function, or operator expression for"
-								" DEFAULT"),
 						 parser_errposition(pstate, exprLocation(expr))));
 			if (contain_var_clause(expr))
 				ereport(ERROR,

src/test/regress/expected/sqljson_queryfuncs.out

@@ -1203,11 +1203,11 @@ ERROR:  DEFAULT expression must not contain column references
 LINE 1: SELECT JSON_QUERY(js, '$'  RETURNING int DEFAULT b + 1 ON ER...
                                                          ^
 SELECT JSON_QUERY(js, '$'  RETURNING int DEFAULT sum(1) over() ON ERROR) FROM test_jsonb_mutability;
-ERROR:  can only specify constant, non-aggregate function, or operator expression for DEFAULT
+ERROR:  can only specify a constant, non-aggregate function, or operator expression for DEFAULT
 LINE 1: SELECT JSON_QUERY(js, '$'  RETURNING int DEFAULT sum(1) over...
                                                          ^
 SELECT JSON_QUERY(js, '$'  RETURNING int DEFAULT (SELECT 1) ON ERROR) FROM test_jsonb_mutability;
-ERROR:  can only specify constant, non-aggregate function, or operator expression for DEFAULT
+ERROR:  can only specify a constant, non-aggregate function, or operator expression for DEFAULT
 LINE 1: SELECT JSON_QUERY(js, '$'  RETURNING int DEFAULT (SELECT 1) ...
                                                          ^
 DROP TABLE test_jsonb_mutability;