Fix process.nextTick throw call sites

This patch now reports the proper throw call site for exceptions
triggered within process.nextTick. So instead of this:

node.js:201
        throw e; // process.nextTick error, or 'error' event on first tick
              ^

You will now see:

mydir/myscript.js:15
  throw new Error('My Error');
          ^

From my testing this patch causes no performance regressions, but does
greatly simplify processing the nextTickQueue.