gracefully handle 500 errors on server requests

[adobeDan]

Things happen sometimes that cause servers to go boom, and user-sync should not choke when that happens (as documented in #114 and #116). Given that most of user-sync's actions are idempotent, probably it should try to retry a batch of actions when they fail, but at worst it should just log the failure and move on as if the operation succeeded. Maybe we even need a setting for what should happen in this case.

[adobeDan]

I'm also gracefully handling ClientErrors (where the umapi_client doesn't understand the server request). In both cases I'm outputting all the error info as CRITICAL and continuing.

[adobeDan]

This was closed in d76c1d5, but I fat-fingered the commit message so it didn't close automatically.