incorporate better error messages in libaml.
check for divide by zero on Omod

Reference: /n/atom/patch/applied2013/amlmsgs
Date: Thu Jun 20 18:54:37 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libaml/aml.c	Thu Jun 20 18:54:08 2013
+++ /sys/src/libaml/aml.c	Thu Jun 20 18:54:08 2013
@@ -835,12 +835,12 @@
 		default:
 			if(PC >= FP->end){
 			Overrun:
-				print("PC overrun frame end");
+				print("aml: PC overrun frame end");
 				goto Out;
 			}
 			FP++;
 			if(FP >= FT){
-				print("frame stack overflow");
+				print("aml: frame stack overflow");
 				goto Out;
 			}
 			*FP = FP[-1];
@@ -1268,7 +1268,7 @@
 {
 	int i;
 
-	print("bad opcode %p: ", PC);
+	print("aml: bad opcode %p: ", PC);
 	for(i=0; i < 8 && (FP->start+i) < FP->end; i++){
 		if(i > 0)
 			print(" ");
@@ -1524,23 +1524,22 @@
 	case Osub:
 		r = mki(ival(FP->arg[0]) - ival(FP->arg[1]));
 		break;
-	case Omod:
-		d = ival(FP->arg[1]);
-		r = mki(ival(FP->arg[0]) % d);
-		break;
 	case Omul:
 		r = mki(ival(FP->arg[0]) * ival(FP->arg[1]));
 		break;
+	case Omod:
 	case Odiv:
 		v = ival(FP->arg[0]);
 		d = ival(FP->arg[1]);
 		if(d == 0){
-			print("aml: divide by 0: %#p\n", PC);
-			return 0;
+			print("aml: division by zero: PC=%#p\n", PC);
+			return nil;
 		}
+		r = mki(v % d);
+		store(r, FP->arg[2]);
+		if((FP->op - optab) != Odiv)
+			return r;
 		r = mki(v / d);
-		if(FP->arg[2])
-			store(mki(v % d), FP->arg[2]);
 		store(r, FP->arg[3]); 
 		return r;
 	case Oshl: