TYPO3 Core

some more meat to the bare bones:

In the ErrorHandler config of any site configuration record, one has the choice between the content of a page, a Fluid template to be rendered, or a full blown PHP Error Handler serving the output for the configured error code.

It is now wanted that integrators can register error handler classes into $GLOBALS['TYPO3_CONF_VARS'] and only the those registered handlers can be selected for the given error code.
One handler class can be registered for several error codes.

Tasks:

- find a place in $GLOBALS['TYPO3_CONF_VARS'] for this new entry
- define and document the API used (suggestion: refer to custom nodeTypes, reuse the mechanics)
- change the plain text input in site configuration edit form to a select only providing the matching error handlers for selection

for reference: NodeType documentation: https://docs.typo3.org/typo3cms/CoreApiReference/ApiOverview/FormEngine/Rendering/Index.html?highlight=rendertype#nodefactory
we will need no priority here, error handlers should be stand alone.