[FEATURE] Allow assigning object getter values in error views

This introduces an additional configuration option objectVariables
which makes it possible to assign object gettable values to
the error Fluid view.

Besides, it completes the validation schema for the
``TYPO3.Flow.error.exceptionHandler.[...].options``
configuration.

Change-Id: I8ec9ff065c012bc8574c3674d6c5cf3970d15764
Releases: master