install: stop building on build failures

Today, we continue building packages even if one failed to build. It's this way because once upon a time @dcoutts was building all of Hackage and keeping building on failure made sense then. @dcoutts and I agree that this probably not the right default so we should
- stop building on failure (when using parallel builds some packages may already be in-flight, that's OK),
- add a `--keep-going` flag that enables the old behavior.

No files or tests are named. Start by locating the install command's package-build failure handling and its command-line option definitions; verify how parallel builds behave, then confirm that failures stop by default and that --keep-going preserves the previous behavior.

Written by the indexing model from the issue text.