fix crash when device is busy

according to libunicap documentation, format.sizes could be NULL.

Signed-off-by: antoine@nusmuk-12.04 <antoine.villeret@gmail.com>
Merge remote-tracking branch 'antoine/fix-videounicap'
use verbose() rather than error()

this makes it less verbose in automatic mode (when we don't want to
see the error messages of all plugins until one of them matches...)