g.extension: show fatal message if installed Add-On doesn't exists in the repository

scripts/g.extension/g.extension.py

@@ -396,7 +396,16 @@ def _get_addons_list(self):
                         )
         return addons_dict
 
+    def _addon_exists(self, addon_list):
+        if not [self.addons[addon] for addon in addon_list if addon in self.addons]:
+            gs.fatal(
+                _("No AddOn named <{}> found in the repository.").format(
+                    ", ".join(addon_list)
+                )
+            )
+
     def fetch_addons(self, addon_list, all_addons=False):
+        self._addon_exists(addon_list)
         if addon_list:
             if self.git_version >= 2.25 and not all_addons:
                 gs.call(

scripts/g.extension/testsuite/test_addons_download.py

@@ -18,6 +18,7 @@
 from pathlib import Path
 
 from grass.gunittest.case import TestCase
+from grass.gunittest.gmodules import SimpleModule
 from grass.gunittest.main import test
 from grass.gunittest.utils import silent_rmtree
 
@@ -155,6 +156,14 @@ def test_github_install_official_multimodule(self):
             if file.suffix != ".html" and file.suffix != ".py":
                 self.assertModule(str(file), help=True)
 
+    def test_github_install_official_non_exists_module(self):
+        """Test installing non exists extension from official addons repository"""
+        extension = "non_exists_extension"
+        gextension = SimpleModule("g.extension", extension=extension)
+        self.assertModuleFail(gextension)
+        self.assertTrue(gextension.outputs.stderr)
+        self.assertIn(extension, gextension.outputs.stderr)
+
 
 if __name__ == "__main__":
     test()