Provide better way to check compile errors.

Now you can just call `mrb_load_*` functions then check
`context->parser_nerr > 0` if the return value is `undef`,
instead of calling `mrb_parse_*` functions independently.
ref #3248 #3331

include/mruby/compile.h

@@ -33,6 +33,8 @@ typedef struct mrbc_context {
   mrb_bool no_exec:1;
   mrb_bool keep_lv:1;
   mrb_bool no_optimize:1;
+
+  size_t parser_nerr;
 } mrbc_context;
 
 MRB_API mrbc_context* mrbc_context_new(mrb_state *mrb);

mrbgems/mruby-compiler/core/parse.y

@@ -5777,6 +5777,7 @@ mrb_load_exec(mrb_state *mrb, struct mrb_parser_state *p, mrbc_context *c)
     return mrb_undef_value();
   }
   if (!p->tree || p->nerr) {
+    c->parser_nerr = p->nerr;
     if (p->capture_errors) {
       char buf[256];
       int n;