some error dialog styling issues

@gaurave, @ussuri, fixes https://github.com/dart-lang/chromedeveditor/issues/2708

- the error dialog now has a max size, and the message will scroll within that if necessary
- fixed some issues with joining the message and exception

before:
![screen shot 2014-07-01 at 4 42 54 pm](https://cloud.githubusercontent.com/assets/1269969/3450536/00422524-017e-11e4-9563-bedb92f94b9f.png)

after:
![screen shot 2014-07-01 at 5 01 23 pm](https://cloud.githubusercontent.com/assets/1269969/3450540/07f38d9e-017e-11e4-8a9f-b3e8b1159fa5.png)