Better error handling

• Adds retries (So we shouldn't get so many 500s from the Circle API breaking things).
• Adds support for nonzero response codes (So that curl failing to even establish a connection to the Circle API won't immediately break things).
• Updates the parameters we check in the Circle response (because the one we were checking no longer has any content, probably due to a Circle bug).
• Allows 404 workflow responses and just doesn't consider them (because they're very old and due to stuck events in Circle's API).