Fix Natron and NatronRenderer exit code.

[acolwell]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Change exit code to 0 when -v/--version or -h/--help are specified. This just makes its behavior more consistent with other applications.

Have you tested your changes (if applicable)? If so, how?

Yes. Installer still builds and passes tests. I also manually verified exit codes with command-lines like "Natron -v; echo $?"

[devernay]

LGTM