fix(installer): improve error handling for deployed flake.nix write

Agent-Logs-Url: https://github.com/naturallaw777/staging_alpha/sessions/b7dfaecc-2b2e-4f5f-bb9a-f97ced90e76e

Co-authored-by: naturallaw777 <99053422+naturallaw777@users.noreply.github.com>

iso/installer.py

@@ -956,12 +956,16 @@ class InstallerWindow(Adw.ApplicationWindow):
                 run(["sudo", "rm", "-rf", path])
 
         GLib.idle_add(append_text, buf, "Writing deployed flake.nix...\n")
-        subprocess.run(
+        proc = subprocess.run(
             ["sudo", "tee", "/mnt/etc/nixos/flake.nix"],
             input=DEPLOYED_FLAKE,
+            capture_output=True,
             text=True,
-            check=True,
         )
+        log(proc.stdout)
+        if proc.returncode != 0:
+            log(proc.stderr)
+            raise RuntimeError(proc.stderr.strip() or "Failed to write deployed flake.nix")
         run(["sudo", "rm", "-f", "/mnt/etc/nixos/flake.lock"])
 
         GLib.idle_add(self.push_create_password)