Make json_error_t transparent again

After looking at the new code for a few days, I didn't like it
anymore. To prepare for the future, a few fields will be added to the
json_error_t struct later.

This reverts commit 23dd078c8d. Some
adjustments were needed because of newer commits.

test/bin/json_process.c

@@ -31,7 +31,7 @@ int main(int argc, char *argv[])
     size_t flags = 0;
 
     json_t *json;
-    json_error_t *error;
+    json_error_t error;
 
     if(argc != 1) {
         fprintf(stderr, "usage: %s\n", argv[0]);
@@ -61,10 +61,7 @@ int main(int argc, char *argv[])
 
     json = json_loadf(stdin, 0, &error);
     if(!json) {
-        fprintf(stderr, "%d\n%s\n",
-                json_error_line(error),
-                json_error_msg(error));
-        free(error);
+        fprintf(stderr, "%d\n%s\n", error.line, error.text);
         return 1;
     }