FPHSA: use NOT_FOUND_MESSAGE in non-config mode
Right now, the ${CMAKE_FIND_PACKAGE_NAME}_NOT_FOUND_MESSAGE
variable is only used in the CONFIG_MODE variant. It'd be nice to have it for find modules as well. I think the _NOT_FOUND_MESSAGE
logic in _FPHSA_HANDLE_FAILURE_CONFIG_MODE
can probably just be moved to _FPHSA_FAILURE_MESSAGE
directly.