[unpackfs] Fix typo

Apparently everyone shipping a squashfs image also has the tools
installed, because the error message reporting that the tools-are-
missing contained a reference to an undefined variable.
Fix that, and while here improve the error message so you
don't get a whole path as a title in the error message.

The slightly weird error-message construction is so that no
messages change and no translation work is needed.

src/modules/unpackfs/main.py

@@ -460,8 +460,9 @@ def run():
             if shutil.which("unsquashfs") is None:
                 libcalamares.utils.warning("Failed to find unsquashfs")
 
-                return (_("Failed to unpack image \"{}\"").format(self.source),
-                        _("Failed to find unsquashfs, make sure you have the squashfs-tools package installed"))
+                return (_("Bad unsquash configuration"),
+                        _("Failed to find unsquashfs, make sure you have the squashfs-tools package installed.") +
+                        " " + _("Failed to unpack image \"{}\"").format(source))
 
     unpack = list()