Preserve whitespace in traceback text

Exception messages sometimes contain detail information in separate lines, for readability in log/console output; this change preserves that readability in Sentry layout.

Example:

![image](https://cloud.githubusercontent.com/assets/153580/4318782/03c923e0-3f23-11e4-8e8b-db90ae0865ff.png)

becomes:

![image](https://cloud.githubusercontent.com/assets/153580/4318802/32c4d536-3f23-11e4-8c62-f9d49eca0931.png)

I've made this layout tweak locally for some time; haven't yet encountered a case in which there was a downside to it.