switch_root: improve behavior on error; improve help text
*: make "can't execute '%s'" message uniform

diff --git a/selinux/runcon.c b/selinux/runcon.c
index e94ff14..6ecd789 100644
--- a/selinux/runcon.c
+++ b/selinux/runcon.c
@@ -129,10 +129,10 @@
 				     context_str(con));
 
 	if (setexeccon(context_str(con)))
-		bb_error_msg_and_die("cannot set up security context '%s'",
+		bb_error_msg_and_die("can't set up security context '%s'",
 				     context_str(con));
 
 	execvp(argv[0], argv);
 
-	bb_perror_msg_and_die("cannot execute '%s'", argv[0]);
+	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
 }