Changes error handler for exception handler

Changes PO\Application\IErrorHandler which handled both errors and
exceptions into simply an IExceptionHandler for just exceptions. This
is coupled with a new instance of said handler which converts all
errors into exceptions using the \ErrorException class.

Other simplifications made to match this new interface.
Adds exception handling to Rest applications

Makes the Rest implementation of Application\IDispatchable accept an
instance of Application\IExceptionHandler and use it when unexpected
exceptions are encountered.

Also adds a rest API friendly instance of the exception handler
interface which shows clear debug information in JSON format.
Merge pull request #2 from peteonline/feat-rest-error-handling

Feat rest error handling