catch errors in bound() methods

I'm using this module in level-browser-fs which uses level-fs which uses IDBWrapper, and the range functions are throwing when I try to `fs.stat()` a directory from the / that doesn't exist:

```
Uncaught Error: DataError: DOM IDBDatabase Exception 0 idbstore.js:1130
```

I'm pretty far up the stack from this module, but wrapping the bound functions in try/catches and returning undefined when they throw seems to make everything work properly far up the stack.