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-get-default.rst b/docs/manpages/ndnsec-get-default.rst
index 48889c8..54da09e 100644
--- a/docs/manpages/ndnsec-get-default.rst
+++ b/docs/manpages/ndnsec-get-default.rst
@@ -8,7 +8,7 @@
 
 ::
 
-    $ ndnsec-get-default [-h] [-kc] [-i identity|-K key] [-q]
+    ndnsec-get-default [-h] [-kc] [-i identity|-K key] [-q]
 
 Description
 -----------