This reverts commit 9d7597d6, reversing
changes made to 6b6390f7.

Added support for different response formats.
Support for error response in different formats.

`$this` -> `$self`

PHP supports exception stacks since 5.3 so we should use it.
Also Errorhandler is now able to display the stack: #297

make sure all information is displayed
Fixes comment by @creocoder issue #408
https://github.com/yiisoft/yii2/pull/408#issuecomment-18454179

class name is alwarys HttpException, better display the name of the http
error.