Parser sets exitcode on errors, but generator does not

In main.cc, the parse() function and the generate() function do behave inconsistently w/regard to exceptions being thrown.

Both functions catch the exception, but ...

• parse() exits with code 1 and prints the error to stderr
• generate() only prints the error to stdout and finally exits with code 0

Proposal is to change generate() similar to the behaviour of parse(). Patch is ready, follows.