`rb_obj_clone` can possibly raise an exception, and the exception isn't
always helpful.  This commit catches the exception and raises an
IsolationError telling the user what went wrong.

Before this commit:

```
> ruby -e'Ractor.make_shareable(->{}, copy:true)'
<internal:ractor>:828:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)
	from -e:1:in '<main>'
```

After this commit:

```
> ./miniruby -e'Ractor.make_shareable(->{}, copy:true)'
-e:1:in 'Ractor.make_shareable': cannot copy #<Proc:0x000000011f311a80 -e:1 (lambda)> (Ractor::IsolationError)
	from -e:1:in '<main>'
-e:1:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)
	from -e:1:in '<main>'
```

[Bug #21451]