Fixed exceptions when entering non-numerical waypoints

[RandomStarfighter]

Previously, entering non-numerical values into a waypoint grid, or deleting a cell without providing a number, would throw an exception.
This avoids the exception and resets the cell value back to zero if an invalid string is entered.
Note: I noticed that XWA used an additional Math.Round() operation, when the other platforms didn't. I didn't know if there was any reason why, so I kept as is.

EDIT: Only realized after the fact, this doesn't occur in release builds... because of the try/catch... because of course.
Well, at least it won't explode when running in the debugger. I got burned by that issue when editing a test mission, so I was compelled to fix it. Not sure if I should keep this or not.

[MikeG621]

Yeah, you must have "always break" turned on. I don't use that feature since there's plenty of "typical" exceptions that get thrown throughout normal operation, so the try/catch handles it, even when I'm running through the debugger.

[RandomStarfighter]

A long time ago there were strange issues of release builds throwing exceptions that the debug build wouldn't. So at some point I became more cautious of exceptions, allowing the debugger to catch more of them, so I could fix the code to prevent any exceptions to begin with. But I hadn't changed my exception rules in a while, and it seemed like a sudden new problem that caught me off guard.

[MikeG621]

So decided to go ahead and merge this and then I'll remove the try/catch block