Modifying frozen objects will raise FrozenError.

`FrozenError` is a subclass of `RuntimeError` which used to be
raised.  [Ruby2.5]

src/array.c

@@ -114,7 +114,7 @@ static void
 ary_modify_check(mrb_state *mrb, struct RArray *a)
 {
   if (MRB_FROZEN_P(a)) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen array");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen array");
   }
 }
 

src/class.c

@@ -430,9 +430,9 @@ mrb_define_method_raw(mrb_state *mrb, struct RClass *c, mrb_sym mid, mrb_method_
 
   if (MRB_FROZEN_P(c)) {
     if (c->tt == MRB_TT_MODULE)
-      mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen module");
+      mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen module");
     else
-      mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen class");
+      mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen class");
   }
   if (!h) h = c->mt = kh_init(mt, mrb);
   k = kh_put(mt, mrb, h, mid);

src/hash.c

@@ -286,7 +286,7 @@ static void
 mrb_hash_modify(mrb_state *mrb, mrb_value hash)
 {
   if (MRB_FROZEN_P(mrb_hash_ptr(hash))) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen hash");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen hash");
   }
   mrb_hash_tbl(mrb, hash);
 }

src/string.c

@@ -494,7 +494,7 @@ static void
 check_frozen(mrb_state *mrb, struct RString *s)
 {
   if (MRB_FROZEN_P(s)) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen string");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen string");
   }
 }
 

src/variable.c

@@ -261,7 +261,7 @@ mrb_obj_iv_set(mrb_state *mrb, struct RObject *obj, mrb_sym sym, mrb_value v)
   iv_tbl *t = obj->iv;
 
   if (MRB_FROZEN_P(obj)) {
-    mrb_raisef(mrb, E_RUNTIME_ERROR, "can't modify frozen %S", mrb_obj_value(obj));
+    mrb_raisef(mrb, E_FROZEN_ERROR, "can't modify frozen %S", mrb_obj_value(obj));
   }
   if (!t) {
     t = obj->iv = iv_new(mrb);