CameraInfoMissingError exception is misleading

[jack-oquin]

This exception indicates that loadCameraInfo() has not been called since the current URL and camera name were defined or changed.

It does not indicate whether the camera info file actually exists, but users will probably mistakenly think the camera is not calibrated. That is not the problem.

I would like to change the name to CameraInfoNotLoadedError, but then any code that catches the former name will no longer work, failing silently.

Perhaps the best solution is to improve the documentation for this exception, which is currently somewhat vague.

See ros-drivers/axis_camera#18 for a long discussion demonstrating the possible confusion.