MM-30041: Return correct error message for user save (#16117)

* MM-30041: Return correct error message for user save

We were collapsing all types of user conflict into a single
error message. Fixed it by inspecting the field of the invalidError type
and returning the correct message.

https://mattermost.atlassian.net/browse/MM-30041

```release-note
NONE
```

* Fix test errors

* Fix wrong message in test when comparing error messages

Co-authored-by: Rodrigo Villablanca <villa061004@gmail.com>

app/bot_test.go

@@ -97,7 +97,7 @@ func TestCreateBot(t *testing.T) {
 			OwnerId:     th.BasicUser.Id,
 		})
 		require.NotNil(t, err)
-		require.Equal(t, "app.user.save.existing.app_error", err.Id)
+		require.Equal(t, "app.user.save.username_exists.app_error", err.Id)
 	})
 }