runtime: make type assertion a runtime.Error, the first of many

R=r
CC=golang-dev
https://golang.org/cl/805043
diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index 6001c22..3dd9977 100644
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -935,7 +935,7 @@
 		printf("\t");
 	}
 	printf("panic: ");
-	printany(p->arg);
+	·printany(p->arg);
 	if(p->recovered)
 		printf(" [recovered]");
 	printf("\n");