Display exit code on build details page for the failed builds

Currently, the build details page doesn't display build process exit code if the build fails which makes debugging in some cases hard.

This pull request changes that and does the following:

* Modify `Build` model to also store build process exit code in the `exit_code` attribute
* Modify build details template to display exit code if the build failed

I intentionally haven't included re-generated and updated `.po` files, because after I ran `makemessages -l en`, I saw a bunch of other changes and I didn't want to pollute this pull request with other changes. If you still want me to include the updated translation files, let me know.