Kernel.loop should not catch StandardError.

Current implementation of Kernel.loop prevents us to catch a StandardError exception raised in the loop block because Kernel.loop itself catches it then throws it away.

```
begin
  loop do
    hoge
  end
rescue => e
  p e
end
```

```sh
% ruby a.rb
#<NameError: undefined local variable or method `hoge' for main:Object>

% bin/mruby a.rb
%
```