In
I have ${sessionScope.AbstractProcessingFilter.SPRING_SECURITY_LAST_EXCEPTION_KEY.message}
you are trying to access session attribute called AbstractProcessingFilter, but it is Spring Security class, which have constant SPRING_SECURITY_LAST_EXCEPTION_KEY, which contains name of exception attribute.
You can get name of this attribute here:
Spring Security API.
You must write: ${sessionScope.SPRING_SECURITY_LAST_EXCEPTION.message} for exception message.