[dice] Ugly SyntaxErrors for invalid die rolls

[dgw]

People really like trying to break bots… Several users got in on the fun this weekend, trying to roll dice like 0x1d0x3, 0dNaN, 1d3d4d5, onedone, and variations on that sort of thing.

The dice module, I guess, doesn't catch SyntaxError exceptions. If parsing the given dice expression fails, some generic error ("I'm sorry, you have to give me real dice." or something like that) should get output rather than the likes of Ast.Node 'Call' not implemented.

Probably an easy fix, which I'll get to soon enough if nobody else beats me to it.