[mount] Bail on bad config

- One might argue whether an empty list of partitions to mount is
   a bad thing. It suggests that the partition module wasn't used,
   and so we're in an OEM situation -- but then everything should
   already be mounted anyway. That's why I choose empty -> bail.

src/modules/mount/main.py

@@ -133,9 +133,15 @@ def run():
 
     :return:
     """
-    root_mount_point = tempfile.mkdtemp(prefix="calamares-root-")
     partitions = libcalamares.globalstorage.value("partitions")
 
+    if not partitions:
+        libcalamares.utils.warning("partitions is empty, {!s}".format(partitions))
+        return (_("Configuration Error"),
+                _("No partitions are defined for <pre>{!s}</pre> to use." ).format("mount"))
+
+    root_mount_point = tempfile.mkdtemp(prefix="calamares-root-")
+
     # Guard against missing keys (generally a sign that the config file is bad)
     extra_mounts = libcalamares.job.configuration.get("extraMounts") or []
     extra_mounts_efi = libcalamares.job.configuration.get("extraMountsEfi") or []