Confirm that options are valid before starting

When using the 'Sampler' or 'Checkpoint' options, if one of the entered
names has a typo, an error will only be thrown once the `draw_xy_grid`
loop reaches that name. This can waste a lot of time for large grids
with a typo near the end of a list, since the script needs to start over
and re-generate any earlier images to finish making the grid.

Also fixing typo in variable name in `draw_xy_grid`.