fixes for Response.writeError with servlet error dispatch

[lachlan-roberts]

See issue #12697

Doing a Response.writeError() will try to invoke the servlet ErrorHandler, which can do an ERROR dispatch. This does not work properly if it is not done from within the scope of the ServletChannel.

• Add checks inside the ErrorHandler implementations to stop dispatching to error pages unless we have already started the ServletChannel.
• Authenticators now use the ServletChannel mechanism to do a sendError() when the ServletChannel starts handling the request. It uses a new static method ErrorHandler.writeError which is overriden by the servlet ErrorHandlers.

[janbartel]

@lachlan-roberts this is quite a hefty refactor of ee10 error handling code, of publically available methods - are you sure we shouldn't just be doing this in ee11 only?

[lachlan-roberts]

@janbartel I have aligned the EE10 ErrorHandler with changes from the EE11 one.

I think maybe the changes can be reduced, but I wanted the EE10 ErrorHandler to extend the core ErrorHandler like the EE11 one does, so that I could introduce this new ErrorHandler.writeError method.

If we really don't want to change the EE10 ErrorHandler I could introduce a new interface which both EE10 and EE11 ErrorHandlers could extend.

[lachlan-roberts]

@gregw @janbartel ready for review