check if target_commitish exists on github before release

[ungoldman]

Currently just 500s like so:

{ tag_name: 'v1.0.2',
  target_commitish: '6065f55b021ee4698fc772d1911181c85d9a9d9d',
  name: 'v1.0.2',
  body: '* double checking refactoring\n\n',
  owner: 'bcomnes',
  repo: 'gh-release-test',
  draft: true,
  prerelease: false }
? does this look right? Yes
{ [Error: {"message":"Server Error","documentation_url":"https://developer.github.com/v3"}]
  message: '{"message":"Server Error","documentation_url":"https://developer.github.com/v3"}',
  code: 500 }

Would be better to check if the commit has been pushed and if not display an informative error message.

5

[ungoldman]

@paulcpederson