no error when module exports non-existing package

[Victor Rudometov]

Hi,
Javac does not complain if user exports a package that does not exist. 
If it was not done intentionally, it would be good to report an error, 
or at least a warning.
Today was the third time I saw the code with a typo or just incorrect 
package name in module-info's exports statement.

Thanks.
Victor.