Fix marker-out-of-bounds crash when defining zone points

In the zone definition view, I can drag markers up to 3 pixels beyond the right and bottom edges of the image.  These values are accepted without validation and cause the zma process to crash loop.

"Zone 9/New for monitor Living extends outside of image dimensions, 0, 0, 642, 482"

This change:
1. Fixes the CSS problem that allowed me to drag elements beyond the bounds of the image
2. Fixes the Javascript so that it will validate drag-based coordinate inputs in case the CSS regresses
3. Reclassifies this situation from Panic -> Error, since there is a reasonable recovery path here (ignore the zone) that is better than crashing.