Provide a more useful error for invalid use case.

The previous help statement was not very usefull for users who
don't know how to use nbconvert (ironically, the exact set of
users who would encounter the error).

IPython/nbconvert/nbconvertapp.py

@@ -215,11 +215,9 b' class NbConvertApp(BaseIPythonApplication):'
 
             # No notebooks were specified, show help.
             if len(self.notebooks) == 0:
-                self.print_help()
+                self.print_examples()
 
-            # Notebooks were specified, but not converted successfully.  Show how
-            # to access help.
-            else:
+            # Show how to access help.
                 print('For help, use "ipython nbconvert --help"')