Save changeset errors and data for GitHub webhooks

[joshsmith]

What's in this PR?

• Passes error data out of marshall_result from GitHub.Sync
• Adds data and error to GithubEvent to:
  • track the data as it was before, if any
  • track the exact error that we received, if any
• Formats the data pretty-printed so we can display it in our admin panel
• Sends an exception to Sentry so we can track that an error happened

Fixes#1192

I would ideally like to:

• Test the Sentry event was sent
• Create a follow-up pull request that simply uses this passing test as the basis for a failing test for the syncing (i.e. this test right now should result in the event being processed)
• Take what was learned here and apply it to Syncing does not match ResultAggregator errors #1148 with similar results, and ideally creating a similar audit trail for whole-repo syncing