Improve error message failing to copy proc

Previously, when passing an unshareable proc to a Ractor we would get
the message:

    'Ractor.new': allocator undefined for Proc (TypeError)

With this change we get:

    'Ractor.new': can not copy unshareable object #<Proc:0x00007f1b31713600 test.rb:2> (Ractor::Error)

Which should hopefully be a little clearer about the problem and make it
simpler to debug.

Author: jhawthorn

bootstraptest/test_ractor.rb

@@ -507,14 +507,14 @@ def test n
 }
 
 # To copy the object, now Marshal#dump is used
-assert_equal "allocator undefined for Thread", %q{
+assert_match /can not copy unshareable object/, %q{
   obj = Thread.new{}
   begin
     r = Ractor.new obj do |msg|
       msg
     end
-  rescue TypeError => e
-    e.message #=> no _dump_data is defined for class Thread
+  rescue Ractor::Error => e
+    e.message
   else
     'ng'
   end

ractor.c

@@ -2075,6 +2075,9 @@ copy_enter(VALUE obj, struct obj_traverse_replace_data *data)
         return traverse_skip;
     }
     else {
+        if (!rb_get_alloc_func(rb_obj_class(obj))) {
+            rb_raise(rb_eRactorError, "can not copy unshareable object %+"PRIsVALUE, obj);
+        }
         data->replacement = rb_obj_clone(obj);
         return traverse_cont;
     }

test/ruby/test_ractor.rb

@@ -213,6 +213,16 @@ def test_ifunc_proc_not_shareable
     assert_unshareable(pr, /not supported yet/, exception: RuntimeError)
   end
 
+  def test_copy_unshareable_object_error_message
+    assert_ractor(<<~'RUBY')
+      pr = proc {}
+      err = assert_raise(Ractor::Error) do
+        Ractor.new(pr) {}.join
+      end
+      assert_match(/can not copy unshareable object/, err.message)
+    RUBY
+  end
+
   def test_ractor_new_raises_isolation_error_if_outer_variables_are_accessed
     assert_raise(Ractor::IsolationError) do
       channel = Ractor::Port.new