Add 500 Error on `View::renderError()`

Add 500 error on `View::renderError()`

When Database fails or Exceptions occur, concrete5 returns a 200 OK
response.

- Add php `header()` call to set 500 header
Merge pull request #1034 from mkly/500_error_on_render_error

Add 500 Error on `View::renderError()`