Catch BigNumber errors in roundedAmount filter

This change structures the code like how the `roundAmount` filter does.

Part 1 of https://github.com/stellar/stellar-client/issues/977
> ## Part 1: Catch BigNumber errors
> ![screen_shot_2014-10-08_at_2_47_10_pm](https://cloud.githubusercontent.com/assets/5728307/4568107/23694ad0-4f37-11e4-8376-e67772bb370e.png)