[Bugs] #3709 Jukebox UNSP: Jukebox should safely ignore when referenced media is not found

[Sugar Labs Bugs]

#3709: Jukebox should safely ignore when referenced media is not found
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    Reporter:  manuq                      |          Owner:  kushal     
        Type:  defect                     |         Status:  new        
    Priority:  Unspecified by Maintainer  |      Milestone:  0.96       
   Component:  Jukebox                    |        Version:  Unspecified
    Severity:  Unspecified                |       Keywords:  12.1.0     
Distribution:  Unspecified                |   Status_field:  Unconfirmed
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 And may also tell the user: "N medias were not found".

 TestCase:

  1. start Jukebox from scratch
  2. add media from a external device, like a pendrive
  3. stop the activity
  4. remove the external device, restart the activity from the Journal
 entry

 Currently, an error is shown in the logs for the second time the activity
 is started.  Log attached.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/3709>
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