Added backtrace error info.

* cradle/exception/backtrace_info.hpp: Added backtrace error info
  interfaces.
* src/exception/backtrace_info.cpp: Added backtrace error info
  implementation.
* src/exception/jamfile: Added dependency on `Boost.Exception`.
* test/unit/exception/backtrace_info_run.cpp: Added unit tests for backtrace
  error info.