async_wrap: don't abort on callback exception

Rather than abort if the init/pre/post/final/destroy callbacks throw,
force the exception to propagate and not be made catchable. This way
the application is still not allowed to proceed but also allowed the
location of the failure to print before exiting. Though the stack itself
may not be of much use since all callbacks except init are called from
the bottom of the call stack.

    /tmp/async-test.js:14
      throw new Error('pre');
      ^
    Error: pre
        at InternalFieldObject.pre (/tmp/async-test.js:14:9)

PR-URL: https://github.com/nodejs/node/pull/5756


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>