single argument panic

note that sortmain.go has been run through hg gofmt;
only the formatting of the day initializers changed.
i'm happy to revert that formatting if you'd prefer.

stop on error in doc/progs/run

R=r
CC=golang-dev
https://golang.org/cl/850041
diff --git a/test/fixedbugs/bug253.go b/test/fixedbugs/bug253.go
index f359961..bb5b770 100644
--- a/test/fixedbugs/bug253.go
+++ b/test/fixedbugs/bug253.go
@@ -20,9 +20,10 @@
 	S3
 	S1
 }
+
 func main() {
 	var s4 S4
-	if s4.i != 0 {	// .i refers to s4.S1.i, unambiguously
-		panic()
+	if s4.i != 0 { // .i refers to s4.S1.i, unambiguously
+		panic("fail")
 	}
 }