Fixed method signature in on_row_expanded_change()
bz19847: workarounds for the issue

I'm not sure how it happened, but it seems that there was a path in
the metadata_status table, that didn't correspond to a filename in the
device_item table.  Made a couple changes to work around this:

  - If we fail creating a DeviceItem in scan_device_for_files(), the
    we log a warning and continue instead of crashing.
  - Before scan_device_for_files() runs, remove data from the metadata
    tables that doesn't have a corresponding filename in device_item