Add error handling to make_pack_deltas and called functions

[matthewrsj]

Reported by @pohly

make_pack_deltas and everything that it calls ignores errors. It can of course proceed without the deltas, but the result isn't what it should be.

For example, suppose there is a download error because of some temporary server failure or some permanent error because the URL was misconfigured. Such errors should not be ignored. I'd expect swupd_make_pack to fail instead. Error handling should be added to at least report and abort when delta pack creation fails.