Update customizing default exception handler section

[KremnevDmitry]

New variables for defining the size of the exception dialog have been introduced. See: https://github.com/jmix-framework/jmix/issues/3177

New CSS variables has been defined:

• --jmix-exception-dialog-width
• --jmix-exception-dialog-expanded-width
• --jmix-exception-dialog-expanded-height

It can be used to style the ExeptionDialog

People need to know about this, this section needs to be updated with new information: https://docs.jmix.io/jmix/flow-ui/exception-handlers.html#customizing-default-handler