Add error message when sr_calibration is not found

[toliver]

If sr_calibration parameter is not found (e.g. when using a wrong the serial number on the hand) we should show a more informative message before the ROS_ASSERT that makes it exit.

https://github.com/shadow-robot/sr-ros-interface-ethercat/blob/indigo-devel/sr_robot_lib/src/sr_robot_lib.cpp#L342