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-cert-gen.rst b/docs/manpages/ndnsec-cert-gen.rst
index 25c6427..97e8e87 100644
--- a/docs/manpages/ndnsec-cert-gen.rst
+++ b/docs/manpages/ndnsec-cert-gen.rst
@@ -8,7 +8,7 @@
 
 ::
 
-    $ ndnsec-cert-gen [-h] [-S timestamp] [-E timestamp] [-I info] [-s sign-id] [-i issuer-id] request
+    ndnsec-cert-gen [-h] [-S timestamp] [-E timestamp] [-I info] [-s sign-id] [-i issuer-id] request
 
 Description
 -----------