[!!!][BUGFIX] Return 404 status code for removed entities
With this change the status code for persisted entities that can't
be found is no longer ``500``.
When an exception is thrown in ``PropertyMapper::convert()`` it is
always wrapped in a ``\TYPO3\Flow\Property\Exception`` leading to the
default exception handling which sets the status code of the HTTP
response to 500.
This is a breaking change only in the case that one relied on the
(incorrect) behavior of returning a status code 500 for entities that
couldn't be found.