Fix #765 and #793. Increase bin url length and posibilities.

The error was:

1. being caused by strings being passed up through the error handler - instead of actual error objects.
2. more importantly, the 10 tries jsbin was attempting to get a new bin id, it was failing!!!! so I've increased the chances by adding uppercase characters (doubling the possibilties) and on each hit on an existing bin, the shortcode will get longer