doc: network issues during fetch lead to empty files

When fetching the latest manual with make -C doc fetch, everything appeared to work correctly. But there are several files that are empty now:

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   debian/changelog
	modified:   doc/manual/en/images/conversations-add-contacts.png
	modified:   doc/manual/en/images/conversations-first-screen.png
	modified:   doc/manual/es/images/apu1d.jpg
	modified:   doc/manual/es/images/apu1d_thumb.jpg
	modified:   doc/manual/es/images/upgrades_es_v01.png
	modified:   doc/manual/es/images/virtualbox_console_after_boot.png

james@thelio:~/salsa/freedombox-team/freedombox$ ls -al doc/manual/en/images/conversations-add-contacts.png 
-rw-r--r-- 1 james james 0 Jan 17 20:02 doc/manual/en/images/conversations-add-contacts.png

I believe this is due to some temporary network issue that occurred during the fetch. Ideally, the fetch command would error out when this occurs, and should not write empty files. It looks like any errors are masked when using wget --quiet.