Camera Match - Error Messages and UI refinement

Implemented proper error messages for the user.
User now gets warned if:
- no image is loaded
- first or second vanish point is missing

Create Camera:
- view switches now to the new camera
- backdrop mode is changed to “camera”
- render resolution is now set to the loaded image

Some naming changes in the UI:
- X and Y-Axis renamed to “first vanish point” and “second vanish point”
- Help link now points to github wiki page