minor: improve error management for missing system gcc compiler in system configuration

[albestro]

In the cluster configuration the file packages.yaml is optional

stackinator/stackinator/recipe.py

Lines 91 to 98 in 5581430

	# load system/packages.yaml -> system_packages (if it exists)
	system_packages = {}
	system_packages_path = self.system_config_path / "packages.yaml"
	if system_packages_path.is_file():
	# load system yaml
	with system_packages_path.open() as fid:
	raw = yaml.load(fid, Loader=yaml.Loader)
	system_packages = raw["packages"]

but at the same time there is an implicit requirement since if there is no gcc specified in packages.yaml, we should raise an exception.

stackinator/stackinator/recipe.py

Lines 100 to 106 in 5581430

	# extract gcc packages from system packages
	# remove gcc from packages afterwards
	if system_packages["gcc"]:
	gcc_packages = {"gcc": system_packages["gcc"]}
	del system_packages["gcc"]
	else:
	raise RuntimeError("The system packages.yaml file does not provide gcc")

With this small PR, the RuntimeError with a nicer and a bit more informative error message will be raised also in case no packages.yaml or it is available but no gcc is specified.

note: I faced this many times when by mistake I ended up passing the wrong path for the cluster configuration, and the path didn't contain the "usual" yaml files.

[bcumming]

Thanks!