[FEATURE] Make failed calibration more informative.

[refmitchell]

Why do you want this feature?
If the extrinsic calibration fails, there's no feedback to tell you what's gone wrong beyond that the chessboard couldn't be found.

Describe the solution you'd like
At the very least it'd be good to show the undistorted extrinsic image so that we can see if there are any obvious problems with the calibration (or any persistent problems).

Describe alternatives you've considered
n/a

Additional context
n/a