Improve error reporting in GAE with boost::leaf

As we discussed, the current error reporting in GAE is not graceful and hard to debug.
After a roughly investigation, it appears that we may not be fully utilizing the capabilities of boost::leaf.

Here are some examples in the boost::leaf repo may help:

https://github.com/boostorg/leaf/blob/801e5e813ff9923d8d610b1b00597226369328d1/example/error_log.cpp#L51-L59
https://github.com/boostorg/leaf/blob/801e5e813ff9923d8d610b1b00597226369328d1/example/error_trace.cpp#L49-L53

or search keywords in the [docs](https://boostorg.github.io/leaf/#tutorial-diagnostic_information)
- context
- verbose_diagnostic_info

Further investigation is needed, and I believe this is a feasible path to explore.