Stop build jobs more gracefully on failure

Currently if one build fails, shrinkwrap kills all other builds which can lead to confusing errors on subsequent runs. I've had for example incomplete git trees (edk2 submodules maybe?), and partial Linux link leading to aarch64-linux-gnu-objcopy: vmlinux.o: file format not recognized error. The workaround is to manually delete partial build directories before continuing, but one will probably need to do that multiple times while trying to fix the original issue, and it's not immediately obvious that this manual clean is needed.

It may be more user-friendly to do like make: when a build fails, wait for other processes to terminate before quitting shrinkwrap.

This is just a suggestion, not a bug report. If you think it makes sense I could try to work on it when I find time.