Make sure to check things exist before actually modifying them

This fixes crashes when a system does not have squashfs-tools or
a grub dir does not exist.

src/modules/grubcfg/main.py

@@ -29,6 +29,9 @@ def modify_grub_default(partitions, root_mount_point, distributor):
     use_splash = ""
     swap_uuid = ""
 
+    if not os.path.exists(default_dir):
+        return ("Directory does not exist", "The directory {} does not exist on
+        the target".format(default_dir))
     if os.path.exists(plymouth_bin):
         use_splash = "splash"
 
@@ -58,10 +61,10 @@ def modify_grub_default(partitions, root_mount_point, distributor):
     with open(default_grub, 'w') as grub_file:
         grub_file.write("\n".join(lines) + "\n")
 
+    return None
 
 def run():
     partitions = libcalamares.globalstorage.value("partitions")
     root_mount_point = libcalamares.globalstorage.value("rootMountPoint")
     distributor = libcalamares.job.configuration["distributor"]
-    modify_grub_default(partitions, root_mount_point, distributor)
-    return None
+    return modify_grub_default(partitions, root_mount_point, distributor)

src/modules/unpackfs/main.py

@@ -123,6 +123,10 @@ class UnpackOperation:
                 fslist = ""
 
                 if entry.sourcefs == "squashfs":
+                    if shutil.which("unsquashfs") == None:
+                        return ("Failed to unpack image", "Failed to find unsquashfs, make sure you have
+                        the squashfs-tools package installed")
+
                     fslist = subprocess.check_output(["unsquashfs",
                                                         "-l",
                                                         entry.source])