Wrap exceptions in std::exception to verbose output on exception.

[ivanstepanovftw]

I am not getting errors thrown as std::string.

Before:

William Safire will walk us through the nuances of badterminate called after throwing an instance of 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >'

After:

William Safire will walk us through the nuances of badterminate called after throwing an instance of 'std::runtime_error'
  what():  failed to open session/7B/hello.bin: No such file or directory