Show an error message when parsing OPML file fails

[OldGrenouille]

Add-on version
Add-on version:3.3 (French)

Describe the bug
When going on the page to import OMPL files ("import flux), 99% of case I get a "Aucune entrée trouvée" (probably "no entry found"). Sometime I am able to import OMPL that I just export using the export button, however that is not always true.

To Reproduce
Steps to reproduce the behavior:

1. Go to "more option" then
2. Click on "Importer des flux" ("import flux")
3. Select a file
4. See error

Expected behavior
basically I was expecting it would works :D.

Extra context
I'm trying to import all my old rss flux of an alternative version of ff that still uses rss to firefox. So basically I export all my bookmarks, extracts all the line that contains RSS, use a bit of regex to convert it to OMPL and in theory... it should work. Doesn't...

I attached a file containing an example of OMPL that doesn't work. (NB: renamed as .txt to pass through the filter of github)
livemarks-opml-test3.xml.txt

[evilpie]

Thanks for providing the OPML file! It contains an unescaped ampersand: ...rss.php?site=fr&amplang=fr"..., which is invalid XML.

You can replace & by & to make the file work.

[evilpie]

I guess we should show an error