[FEATURE] Make the renderStatically function public

This change makes the renderStatically method of the
DebugExceptionHandler public and returns the output
instead of directly echoing it. This enables other handlers
like the NodeWrappingHandler to render a whole Exception
and output that.

needed by: https://review.typo3.org/#/c/27765/
releases: master

Change-Id: I4eda32a8252e742911ad52b01f28afb67db1d1fc