Intrinsics calibration file existence check

[stonier]

Current behaviour if no intrinsics file exists:

[ERROR] [camera_calibration_parsers]: Unable to open camera calibration file [/home/danielstonier/.ros/camera_info/camera.yaml]
[INFO] [ueye]: UEye: initialised.

Q: Is it required to load an intrinsics file when launching the node?

If yes, then nothing to do. If not, then either:

• Check for existence before calling camera_calibration_parsers::readCalibration, handle it in ueye_cam
• Send a patch upstream that e.g. throws exceptions so users can decide what to do with errors themselves

Actually, the latter would be good regardless of whether it's required or not. Let the user handle the error response in the way they require.

[stonier]

Skipping camera calibration with intrinsics if the file is not found and logging a warning (as opposed to an error).

It should be an acceptable use case to use the stream without calibration (i.e. this should not constitute an error).