Call stack logs should be emitted at TRACE level

[EliAndrewC]

Moved here from the (now private) original Sideboard issue tracker.

One of the cool things that pytest does is when an exception occurs in a unit test, it can spit out a complete call stack. Not just a stack trace, but even all of the local variables that exist at every step of the stack.

This is probably something we can pull out of pytest and use for Sideboard exceptions. This is WAY too verbose to be turned on by default, not to mention dangerous in terms of possibly logging passwords in memory and such. But it would be super-useful to have the option. One way to do this would be to do the logging at TRACE level so that even DEBUG wouldn't get it.

Even if we can't pull it out of pytest, it wouldn't be that hard to implement using the same introspection tools they're using. The trick would be making it compatible with both Python 2 and 3 and ideally turning it off for non-CPython interpreters such as Jython or IronPython where sys._getframe might not be available.