ZXing's decodeFromVideoDevice fires its callback on every video frame
(~30fps). When no barcode is in view, it passes `(null, err)` where err
is 'No MultiFormat Readers were able to detect the code' — this is
NORMAL, not a hardware failure.
The error branch now filters out 'multiformat' and 'unable to detect'
messages, only calling handleCameraError for real device-level issues
(permission denied, camera not found, stream broken).
shawn
111008a858