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).
Moved print description and examples into default help output
Worked with @ivanov
Unnecessary
Return failure, remove comments
Merge pull request #3759 from jdfreder/help_fix

nbconvert: Provide a more useful error for invalid use case.

We now print the help message if no notebooks were converted