doc: Error out properly when a generator fails

In make only timestamps and return values matter. And the shell will create a
file and update its timestamp when it sees an output redirection. No matter if
the program driving the redirected data fails or not.

So even if the first generator run fails, rerunning make will work, with things
looking like they worked out just fine.

This fixes the issue by running the separate steps toward file generation one
by one.

This issue was brought up by Edward E. (cbrt64 on github) in PR#135 on github¹,
including a proof-of-concept implementation, as well as valuable feedback. This
implementation was derived from it. See the pull-request log for details.

¹ https://github.com/grml/grml-etc-core/pull/135