[FEATURE] Make the renderStatically function public (Revision 3)

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.

This change is needed by this change:
https://review.typo3.org/#/c/27765/

Releases: master
Change-Id: I4eda32a8252e742911ad52b01f28afb67db1d1fc
Revision-Id: f5a9efc2961b4118a7141341dc61928fffc2b8ef