Added a piece of code to handle smaller than expected image frames. The frame is lost, but at least the node does not crash.

set ros::Time::now() because libuvc does not set capture_time (https://github.com/ktossell/libuvc/blob/master/src/stream.c#L1100)

http://build.ros.org/view/Ldev/job/Ldev__libuvc_ros__ubuntu_xenial_amd64/3/warnings21Result/

[libuvc_camera/src/camera_driver.cpp] use frame's capture time for timestamp of ros message instead of callback time

The user can change AE mode, and the camera auto-updates
absolute exposure time and white balance temperature, which
are sent on to dynamic_reconfigure clients.