When running with `STEPDEBUG=1`, a response with a `StackTracedError`
would result in a nil pointer error. This commit fixes the check and
adds a test case.
* api/log: initial implementation of the package
* api: refactored to support api/log
* scep/api: refactored to support api/log
* api/log: documented the package
* api: moved log-related tests to api/log