debug(notifications): Improve debug logging for notification callbacks

[luispollo]

I added quite a lot of logging around notification callbacks when I initially rolled this out, but obviously not in the one place where we're seeing an issue now. 😝

This PR adds debug logging around building the proxy request to echo on callbacks, and also adds a bit of additional error handling for retrofit errors such that we hopefully return a better response in error cases.

[spinnakerbot]

The following commits need their title changed:

• 0dd0366: debug(notifications): Improve debug logging for notification callbacks

Please format your commit title into the form:

<type>(<scope>): <subject>, e.g. fix(kubernetes): address NPE in status check

This allows us to easily generate changelogs & determine semantic version numbers when cutting releases. You can read more about commit conventions here.