Image build failures should be fatal

I've seen the following steps fail during image build, but the process has continued regardless:

1. Patch failures (patch does not apply), e.g.:

error: patch failed: lib/middleware/anonymous_cache.rb:301
error: lib/middleware/anonymous_cache.rb: patch does not apply

2. DNS/git clone failures:

I've seen a VM fail to resolve github.com, meaning we couldn't 'git clone' the repo, but this didn't abort the image build

3. Gem fetch failures:

I've seen a VM fail to download and install some ruby gems, but the process continues regardless.

It would be a much simpler process to debug if these errors were very obvious and fatal, rather than requiring an SRE to read hundreds or thousands of lines of output in order to judge whether the built image is likely to work.