[MM-34725] app/user: check if username or email is in use before patching a user (#17392)

* app/user: check if username or email is in use before patching a user

* reflect review comments

* fix tests

app/bot_test.go

@@ -204,7 +204,7 @@ func TestPatchBot(t *testing.T) {
 
 		_, err = th.App.PatchBot(bot.UserId, botPatch)
 		require.NotNil(t, err)
-		require.Equal(t, "app.user.update.find.app_error", err.Id)
+		require.Equal(t, "app.user.save.username_exists.app_error", err.Id)
 	})
 }