Improve error logging in signwebhook + lookup failure interactions

[filmaj]

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.