use SystemStackError instead of RuntimeError

include/mruby.h

@@ -385,6 +385,7 @@ void mrb_print_error(mrb_state *mrb);
 #define E_SYNTAX_ERROR              (mrb_class_get(mrb, "SyntaxError"))
 #define E_LOCALJUMP_ERROR           (mrb_class_get(mrb, "LocalJumpError"))
 #define E_REGEXP_ERROR              (mrb_class_get(mrb, "RegexpError"))
+#define E_SYSSTACK_ERROR            (mrb_class_get(mrb, "SystemStackError"))
 
 #define E_NOTIMP_ERROR              (mrb_class_get(mrb, "NotImplementedError"))
 #define E_FLOATDOMAIN_ERROR         (mrb_class_get(mrb, "FloatDomainError"))

src/error.c

@@ -455,4 +455,5 @@ mrb_init_exception(mrb_state *mrb)
   mrb->nomem_err = mrb_obj_ptr(mrb_exc_new_str(mrb, runtime_error, mrb_str_new_lit(mrb, "Out of memory")));
   script_error = mrb_define_class(mrb, "ScriptError", mrb->eException_class);                /* 15.2.37 */
   mrb_define_class(mrb, "SyntaxError", script_error);                                        /* 15.2.38 */
+  mrb_define_class(mrb, "SystemStackError", exception);
 }

src/vm.c

@@ -160,7 +160,7 @@ stack_extend_alloc(mrb_state *mrb, int room, int keep)
      to prevent infinite recursion. However, do this only after resizing the stack, so mrb_raise has stack space to work with. */
   if (size > MRB_STACK_MAX) {
     init_new_stack_space(mrb, room, keep);
-    mrb_raise(mrb, E_RUNTIME_ERROR, "stack level too deep. (limit=" TO_STR(MRB_STACK_MAX) ")");
+    mrb_raise(mrb, E_SYSSTACK_ERROR, "stack level too deep. (limit=" TO_STR(MRB_STACK_MAX) ")");
   }
 }