Display R errors without Python traceback

Errors from R magic cells currently produce a big Python traceback that's irrelevant to the user:

![rmagic_err_before](https://f.cloud.github.com/assets/327925/726055/af6239c2-e0c8-11e2-9227-708a29d4912d.png)

With these changes, the user only sees the R error, without our traceback:

![rmagic_err_after](https://f.cloud.github.com/assets/327925/726056/c3ef54e2-e0c8-11e2-9ece-7e1689df5230.png)