docs+tools: improve ndnsec help, error messages, and manual page

Exit with status code 2 on malformed command line

Refs: #3777
Change-Id: Id682a5d69c15c74cfa1903db38b5389936f5be58
diff --git a/docs/manpages/ndnsec-sign-req.rst b/docs/manpages/ndnsec-sign-req.rst
index 67d11f4..90a0694 100644
--- a/docs/manpages/ndnsec-sign-req.rst
+++ b/docs/manpages/ndnsec-sign-req.rst
@@ -8,7 +8,7 @@
 
 ::
 
-    $ ndnsec-sign-req [-h] [-k] name
+    ndnsec-sign-req [-h] [-k] name
 
 Description
 -----------