Replace assert by error message to keep library functional

[cehberlin]

No description provided.

[bmagyar]

This potentially leaves transformations broken. How'd you fix that?

[cehberlin]

Dear bmagyar,
sorry for the late response, but I was busy with other things before I was able to check this once more.
For me your library is working even with a empty camera_frame and reference_frame. I have just updated the branch in order to better reflect this in your original assert().

[bmagyar]

Thanks!