Don't delete Tracks from Cozic

[clochix]

Users are complaining that their tracks in Cozic have disappeared. It looks like when we start Music, it imports all tracks from Cozic and try to delete them (in fact, it delete the binary from the document, but the Track document don't seem to be deleted).

Deleting contents without event a warning is very unfair to the users. I now that Cozic is no more maintained now, but if some prefers using Cozic instead of Music, we shouldn't force them to migrate to Music.

[flyingrub]

Normaly the old cozic document is deleted too. But it's right that we need to warn the user that we will transfer all the old file. :)

[clochix]

I have seen two servers where the Track documents have not been deleted, but have no binary attached. This makes Music crash next time it starts and try to import documents from Cozic (fixed in #58).

We definitively need to improve error management in the application:

• handle error case (the code should check if err is not null);
• find a way to report client errors on server side ( see Add a method to log client errors on server cozy/cozy-browser-sdk#19 )