add render_after_error

[janbiedermann]

as with lap20 the after_error callback is available, resembling react 16 error handling
https://reactjs.org/docs/error-boundaries.html#introducing-error-boundaries
a render_after_error block became possible.

[catmando]

I assume we mean like this:

class UserIndex < Hyperloop::Component
  render do
    UL do
      Users.each do |user|
        DisplayUser(user: user)
        .after_error do
          DIV { IMG(src: 'sadface.png') }
        end
     end
   end
end