Improve error logging in signwebhook + lookup failure interactions

Recently saw the following action activations related to someone signing the CLA:

1. First the `signwebhook` is triggered, and the result was:

```
"body": "No usernames were returned from lookup action; maybe an error with lookup action"
```

The start time for this activation was GMT: Friday, October 18, 2019 1:52:04.348 PM
2. The `lookup` action presumably related to the above activation (at GMT: Friday, October 18, 2019 1:52:05.009 PM) returned the following:

```
"error": {},
"reason": "Error during lookup of agreements using API version v6 (params: {\"agreements\":\"CBJCHBCAABAAgYxTJW2t_OytnitWzEgNz3dUYoniQjA8\",\"apiVersion\":\"v6\"})."
```

So overall not enough information in this error case was provided to properly debug the issue. Should improve the logging in this flow.

Start by tracing the signwebhook and lookup actions using the two correlated activation records in the issue. Review how lookup failures are passed to signwebhook, then verify that the resulting logs preserve enough underlying error information and request context to diagnose the failed agreement lookup.

Written by the indexing model from the issue text.