Custom 500 error page or problem description instead

Hello!

I have another problem to solve. Could you please advise? Problem is, when I'm specifying incorrect parameter value, say, &until=1q:00_20090310, I'm getting 500 error without any information on what went wrong. Well, I know that specifying wrong value is bad, but it would be very useful to find out what is actually wrong.
So the question is - can I redefine 500 error behavior somehow, or maybe there's a way to configure Graphite the way it would output error info instead of 500 error?
P.S.: I tried setting Debug = true, but that solves only part of the problem:
1) it gives a lot of excessive info
2) it lets me approximately identify the problem after some reading, but it doesn't have exact error explanation like "URL parameter 'until' contains incorrect value '1q:00_20090310'"

Thanks!