Use an Exception to send data to error_output()

Add a new ErrorHandlerException class, extending ErrorException and used
as a vector to pass error datan from error_handler() to error_output().
This is a "dummy" exception (i.e. not meant to be thrown).

Add new public method getErrorType() to MantisException.

Change error_output() to accept a Throwable (MantisException or
ErrorHandlerException) as parameter, instead of 5 individual values.

This simplifies error_exception_handler() and error_handler(). The
latter creates an ErrorHandlerException based on the error and adapts it
according to its type and the context.

Fixes 0036812