misc: don't crash on duplicated tracks with missing files

nonstop/models.py

@@ -74,11 +74,17 @@ class Track(models.Model):
                 ).exclude(on_air=False).order_by('-play_timestamp')
 
     def file_path(self):
-        return self.nonstopfile_set.all().order_by('creation_timestamp').last().get_local_filepath()
+        nfile = None
+        for nfile in self.nonstopfile_set.all().order_by('creation_timestamp'):
+            if os.path.exists(nfile.get_local_filepath()):
+                return nfile.get_local_filepath()
+        if nfile:
+           return nfile.get_local_filepath()
+        return None
 
     def file_exists(self):
         try:
-            return os.path.exists(self.nonstopfile_set.order_by('creation_timestamp').last().get_local_filepath())
+            return os.path.exists(self.file_path())
         except AttributeError:
             return False