Fix gen_pkg_func failure not being recorded by generate_installers

[andre-rosa]

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 .

[allenh1]

@andre-rosa I don't think I'm quite understanding what this patch does. Can you explain what your concerns are for running without this/what this blocks?

[andre-rosa]

@andre-rosa I don't think I'm quite understanding what this patch does. Can you explain what your concerns are for running without this/what this blocks?

@allenh1 When get_pkg_func() raises NoPkgXml it returns None, [] (no installer, no missing dependency) and that gets recorded as a success generating the installer by generate_installer().

Patch handles the situation if not current and not preserve_existing: record failure. In that case, it'll not output ok('Successfully generated an installer for package ...') but instead err('Failed to generate an installer for package ...').

Every time we don't return an installer it's a failure except when we want to preserve_existing.