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-export.rst b/docs/manpages/ndnsec-export.rst
index dc94939..7b51ad4 100644
--- a/docs/manpages/ndnsec-export.rst
+++ b/docs/manpages/ndnsec-export.rst
@@ -6,7 +6,7 @@
 
 ::
 
-    $ ndnsec-export [-h] [-o output] identity
+    ndnsec-export [-h] [-o output] identity
 
 Description
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