Hook both Exception#initialize and Exception#set_backtrace

Fixes https://github.com/charliesome/better_errors/issues/208.

Will still not work for Exception classes that do not call initialize when using RBX as identified in https://github.com/charliesome/better_errors/issues/155, but should at least work with all previously captured exceptions and all the standard ones in RBX.

Please advise if the style or approach is not to your taste.