force `subprocess.run` to check return code is 0

In general, this should help to capture unintentional errors.

Where the change has created failures, they have been fixed. (Mostly because the git repository that was looked for was missing.)

tests.py

@@ -60,8 +60,7 @@ def test_prepare_environment(tmp_path):
     result = mod.prepare_environment(
         mpsd_release=mpsd_release_to_test, script_dir=script_dir
     )
-    # wait for 20 seconds for the git clone to finish
-    # time.sleep(20)
+
     # check if the directory now is created
     assert release_base_dir.exists()
     # check for spack-environments directory