Fix for issue 1203: rubrics_controller correclty udpates weights, and _errors_list no longer throws an exception

This is a fix for https://github.com/MarkUsProject/Markus/issues/1203

The _errors_list partials were throwing exceptions since the .each_line method doesn't for ActiveRecord::Errors. Furthermore, rather than simply cast all values to floats, with strings resulting in a weight of 0, it allows the model to handle the input error.

![screen shot 2013-10-03 at 1 06 51 am](https://f.cloud.github.com/assets/817212/1259536/f03c381c-2beb-11e3-8b36-3cfb0ceedce8.png)

![screen shot 2013-10-03 at 12 44 10 am](https://f.cloud.github.com/assets/817212/1259537/f03bc62a-2beb-11e3-8d95-bd6046d3550a.png)