Inform property to the user when an import fails, instead of only relying on the browser console

[vitaliylag]

Steps to reproduce:

1. Open about:preferences#search
2. Restore to default
3. Export "Google"
4. Delete "Google"
5. Import "Google"

Expected: Google is imported.

Actual: Google is not imported, nothing's changed.

[nohamelin]

Hi,
It's a limitation mentioned in the add-on description on AMO [edit: I changed a bit the wording of this mention in AMO].

Currently, after your step 5, a related error message is left in the Browser Console. Sure, it would be better if the add-on could inform to the user directly about the problem. I have left pending that work because I'm still not sure how the add-on should show multiple error messages when a bunch of XML files are imported at the same time.