Fix bug where op tag-release on repo with no releases breaks

[emmahsax]

What

See #129 for more information. Just simply rescue that error, and then op tag-release continues how a user would expect.

Why

This will fix #129.

Deploy Plan

Does Platform Operations need to know anything special about this deploy? Are migrations present?

Rollback Plan

• To roll back this change, revert the merge with: git revert -m 1 MERGE_SHA and perform another deploy.

URLs

Links to bug tickets or user stories.

QA Plan

• Run GLI_DEBUG=true ../octopolo/bin/op tag-release on a repository with no releases (for SE folk, I made https://github.com/sportngin/test-emma-release/releases to test with), and watch it succeed

[emmahsax]

QA

$ GLI_DEBUG=true ../octopolo/bin/op tag-release
You are creating the first GitHub release for this repository.
Which version section do you want to increment?
1) Major
2) Minor
3) Patch
> 2
git add CHANGELOG.markdown
git commit -m 'Updating Changelog for 0.1.0'
[master add90fe] Updating Changelog for 0.1.0
 1 file changed, 1 insertion(+)
git tag 0.1.0
git push origin master
git push --tag

[production-status-check]

Has QA approval

[production-status-check]

Needs QA approval

[anfleene]

Can you add a test for this failure mode?

[production-status-check]

Has QA approval