Occasional signwebhook error, how to handle?

Occasionally the signwebhook errors with something along the lines of:

```
"body": "Error retrieving GitHub API instance on behalf of app installation for hubblestack: {\"message\":\"Validation Failed\",\"errors\":[{\"message\":\"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.\",\"resource\":\"Search\",\"field\":\"q\",\"code\":\"invalid\"}],\"documentation_url\":\"https://developer.github.com/v3/search/\"}\nError retrieving GitHub API instance on behalf of app installation for AdobeDocs: {\"message\":\"Validation Failed\",\"errors\":[{\"message\":\"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.\",\"resource\":\"Search\",\"field\":\"q\",\"code\":\"invalid\"}],\"documentation_url\":\"https://developer.github.com/v3/search/\"}\nError retrieving GitHub API instance on behalf of app installation for adobe: {\"message\":\"Validation Failed\",\"errors\":[{\"message\":\"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.\",\"resource\":\"Search\",\"field\":\"q\",\"code\":\"invalid\"}],\"documentation_url\":\"https://developer.github.com/v3/search/\"}\nError retrieving GitHub API instance on behalf of app installation for magento: {\"message\":\"Validation Failed\",\"errors\":[{\"message\":\"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.\",\"resource\":\"Search\",\"field\":\"q\",\"code\":\"invalid\"}],\"documentation_url\":\"https://developer.github.com/v3/search/\"}",
```

The key bit is this:

> The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.

- [ ] First, we should improve this error reporting by also reporting back which user was searched.
- [ ] As a future/second phase, if it DOES turn out to be something around a bad username provided, such as one that does not exist, could we validate this is the case by first ensuring all usernames provided in the agreement exist before attempting to work with GitHub's search API?

Start by locating the signwebhook handling and the GitHub Search API call described in the issue. Confirm where the GitHub API instance errors are assembled; done means the error identifies the searched user, with the optional second phase validating agreement usernames before search.

Written by the indexing model from the issue text.