Fix gen_pkg_func failure not being recorded by generate_installers (#187)
Fix gen_pkg_func failure not being recorded by generate_installers (#187)

generate_installers() didn't consider a few gen_pkg_func() failure
situations like:

  - InvalidPackage:
    The manifest contains invalid XML: syntax error: line 1, column 0
    !!!! 61.7%: Failed to generate installer for package 'qb_chain'!
  - NoPkgXml: Could not fetch pkg!

Those are signaled by gen_pkg_func by returning no installer (None)
and no missing dependencies ([]).

Whenever gen_pkg_func() returns no installer, it must be an error except
when we want to preserve_existing.

Fixes #186 .
ros-infrastructure/superflore,ros-infrastructure/superflore