Always call erl_ddll:format_error on driver load failure

erl_ddll:load_driver usually returns a cryptic error code such
{open_error,-10}.  It's necessary to call erl_ddll:format_error to get
the actual error message, which will usually be rather precise and point
out the problem.
Merge pull request #187 from legoscia/driver-load-errors

Always call erl_ddll:format_error on driver load failure