chromium: fix unsubstituted placeholders in .desktop

like `@@EXTRA_DESKTOP_ENTRIES@@` and `@@URI_SCHEME@@` and add a simple
check to prevent this from happening again.

pkgs/applications/networking/browsers/chromium/browser.nix

@@ -62,19 +62,20 @@ mkChromiumDerivation (base: rec {
       $out/share/applications/chromium-browser.desktop
 
     substituteInPlace $out/share/applications/chromium-browser.desktop \
-      --replace "@@MENUNAME@@" "Chromium" \
-      --replace "@@PACKAGE@@" "chromium" \
-      --replace "Exec=/usr/bin/@@USR_BIN_SYMLINK_NAME@@" "Exec=chromium"
-
-    # Append more mime types to the end
-    sed -i '/^MimeType=/ s,$,x-scheme-handler/webcal;x-scheme-handler/mailto;x-scheme-handler/about;x-scheme-handler/unknown,' \
-      $out/share/applications/chromium-browser.desktop
+      --replace-fail "@@MENUNAME@@" "Chromium" \
+      --replace-fail "@@PACKAGE@@" "chromium" \
+      --replace-fail "/usr/bin/@@USR_BIN_SYMLINK_NAME@@" "chromium" \
+      --replace-fail "@@URI_SCHEME@@" "x-scheme-handler/chromium;" \
+      --replace-fail "@@EXTRA_DESKTOP_ENTRIES@@" ""
 
     # See https://github.com/NixOS/nixpkgs/issues/12433
-    sed -i \
-      -e '/\[Desktop Entry\]/a\' \
-      -e 'StartupWMClass=chromium-browser' \
-      $out/share/applications/chromium-browser.desktop
+    substituteInPlace $out/share/applications/chromium-browser.desktop \
+      --replace-fail "[Desktop Entry]" "[Desktop Entry]''\nStartupWMClass=chromium-browser"
+
+    if grep -F '@@' $out/share/applications/chromium-browser.desktop ; then
+      echo "error: chromium-browser.desktop contains unsubstituted placeholders" >&2
+      exit 1
+    fi
   '';
 
   passthru = { inherit sandboxExecutableName; };